Ep 40 — Time-Resolved Infrared Spectroscopy: Tracking Reaction Kinetics

Series: Infrared Spectroscopy Encyclopedia: From Principles to Practice
Chapter: Part IV · Advanced — Frontier Technologies (Episode 5, concluding the first half of the advanced section)
Target Audience: Photochemistry and photocatalysis researchers, graduate students in catalytic reaction kinetics, electrochemists and in-situ characterization practitioners, reaction mechanism scholars
Prerequisites: Ep 12 (Michelson Interferometer), Ep 14 (ATR Principles), Ep 38 (Synchrotron Radiation Sources), Ep 36–39 (Microscopy and Frontier Techniques)
Reading Time: Approx. 50 minutes

Introduction: Capturing Fleeting Intermediates

In 2017, Friedrich Bartsch's team at the Helmholtz-Zentrum Berlin (HZB) published a study on photocatalytic water splitting in Angewandte Chemie [1]:

"Using step-scan FTIR with ~200 ns time resolution, we captured the formation and decay of the Co(III)-OH intermediate on a hematite photoanode — a species that lives for only ~50 microseconds before converting to O₂. Without time-resolved IR, this key intermediate would remain invisible, leaving the water-splitting mechanism a black box."
—— Bartsch F et al. Angew Chem Int Ed 2017 [1]

This passage succinctly expresses the core value of Time-Resolved FTIR (TR-FTIR): Key intermediates in chemical reactions often have extremely short lifetimes (nanoseconds to milliseconds); only a spectroscopy with a sufficiently fast "shutter" can capture them [1][2]. Conventional FTIR scanning times are on the order of seconds; for many fast reactions, it can only see the "start" and "end", missing the most critical "process".

This episode concludes the first half of the advanced section. We will systematically explain the two modes of time-resolved FTIR (rapid-scan and step-scan), the leap in time resolution from milliseconds to nanoseconds, strategies for capturing reaction intermediates, and reveal how TR-FTIR unveils the "black box" of reaction kinetics through two frontier cases: photocatalytic water splitting and in-situ electrochemical FTIR (SEC-FTIR).

1. Why "Time-Resolved" Infrared?

1.1 Time Scale Limitations of Conventional FTIR

For conventional FTIR, the time for one complete scan (moving mirror traverses a full stroke) depends on [2][3]:

• Moving mirror speed: typically 0.5–5 cm/s (up to 10 cm/s for Bruker VERTEX series from Veeco);
• Spectral resolution: 4 cm⁻¹ resolution requires a moving mirror travel of 0.25 cm;
• Single scan time: ~0.1–1 s (higher resolution requires longer time);
• Multiple scan averaging: typically 16–128 scans, total time 2–60 s.

Problem: The characteristic times of many important chemical processes are far shorter than 1 second [1][2][3]:

Table 1: Chemical reaction time scales and FTIR time resolution capabilities (data sources: Bartsch Angew Chem 2017 [1]; Hessling Vib Spectrosc 2014 [2])

1.2 Two Paths for Time-Resolved Infrared

To cover different time scales, TR-FTIR has developed two core modes [2][3][4]:

   Time Resolution         Technical Path
   ─────────────────────────────────────────
   millisecond (10⁻³ s)  → Rapid-Scan mode
   microsecond (10⁻⁶ s)  → Step-Scan mode
   nanosecond (10⁻⁹ s)   → Step-Scan + repetitive excitation
   picosecond (10⁻¹² s)  → Pump-probe ultrafast spectroscopy (synchrotron FEL)

This episode focuses on the first two (rapid-scan and step-scan), which are standard functions of commercial FTIR instruments; picosecond ultrafast spectroscopy requires large facilities like synchrotron-based FEL and will be briefly mentioned in the frontier direction [2][3][4].

2. Rapid-Scan Mode (Rapid-Scan FTIR)

2.1 Principle: Making the Moving Mirror "Fly"

The core idea of rapid-scan mode: Increase the moving mirror speed to shorten the single-scan time [2][3]:

   Conventional scan: moving mirror speed 0.5 cm/s → single scan ~1 s (4 cm⁻¹)
   Rapid scan: moving mirror speed 10 cm/s → single scan ~50 ms (4 cm⁻¹)
                                           → ~20 ms (16 cm⁻¹, lower resolution)

The moving mirror speed is limited by mechanical stability — the mirror must maintain sub-micrometer alignment accuracy during motion; excessive speed can cause vibration and misalignment. The Bruker VERTEX 80v uses an air-bearing moving mirror, achieving a maximum speed of ~12 cm/s, with single scans as low as ~10 ms [3][4].

2.2 Time Resolution

The time resolution of rapid-scan = single scan time [2][3]:

Table 2: Rapid-scan time resolution (data source: Bruker VERTEX 80v technical specifications [3])

Key trade-off: Resolution and time resolution are inversely proportional — faster scanning requires lower spectral resolution. For many kinetic studies, 16 cm⁻¹ is sufficient to resolve major characteristic peaks [2].

2.3 Data Acquisition and Processing

Typical workflow of rapid-scan [2][3]:

   1. Trigger reaction (e.g., light pulse, potential step, mixing)
        ↓ time t=0
   2. FTIR continuously collects interferograms, moving mirror reciprocates
        · Each reciprocation yields 1 spectrum
        · Acquisition interval = single scan time
        ↓
   3. Synchronously record reaction trigger time and time stamp for each spectrum
        ↓
   4. Fourier transform to obtain time series spectra
        · S(ν, t) — absorbance at wavenumber ν, time t
        ↓
   5. Kinetic fitting
        · Extract A(t) curve for a characteristic peak
        · Fit A(t) = A₀ + A₁·exp(-t/τ₁) + A₂·exp(-t/τ₂)
        · Obtain reaction rate constant k = 1/τ

2.4 Application Scenarios

Rapid-scan is suitable for millisecond to second reactions [2][3][5]:

• Enzyme-catalyzed reactions: steady-state kinetics after substrate mixing;
• Polymer curing: real-time monitoring of epoxy, polyurethane curing;
• Catalyst adsorption/desorption: temperature-programmed desorption of probe molecules (CO, NH₃, pyridine);
• Electrochemical reactions: SEC-FTIR for slow redox processes;
• Photochromism: slow isomerization of spiropyrans, azobenzenes.

3. Step-Scan Mode (Step-Scan FTIR)

3.1 Breaking the Limitation of Moving Mirror Velocity

The "ceiling" of rapid scan is the mechanical speed of the moving mirror—no faster than ~10 ms. To reach microsecond or even nanosecond scales, a different approach is needed [2][3][4]:

   Rapid scan: moving mirror moves continuously, one scan = one time point

   Step-scan: moving mirror moves "step by step", multiple samplings at each position
      · Mirror stops at position x₁ → reaction triggered → acquire time series I(t) at x₁
      · Mirror moves to x₂ → reaction triggered again → acquire I(t) at x₂
      · … until all positions are scanned
      · Recombination: combine all position data at each time point to obtain the interferogram at that time
      · Fourier transform: one spectrum per time point

Key prerequisite: The reaction must be reproducibly excitable—each mirror position requires a reaction trigger. This requires the reaction to be a repeatable process such as photoinduced, electrical pulse triggered, or rapid mixing initiated [2][3][4].

3.2 Time Resolution

The time resolution of step-scan is determined by detector response speed and ADC sampling rate, no longer limited by mirror velocity [2][3][4]:

Table 3: Step-scan time resolution (data sources: Bruker Vertex Series Manual [3]; Hessling Vib Spectrosc 2014 [2])

Practical limit: Commercial FTIR + high-speed MCT + step-scan can achieve time resolution of ~10–100 ns; synchrotron FEL + special detectors can reach picosecond levels [2][4].

3.3 Workflow of Step-Scan

   Reaction trigger (e.g., pulsed laser)
        ↓ t=0
   ┌──────────────────────────────────────────────┐
   │ Mirror position x₁                            │
   │   Reaction trigger → ADC acquires I(t₁), I(t₂), … at x₁ │
   │   Sampling points: t₁=10 ns, t₂=100 ns, …, tₙ=10 ms    │
   │   (Same position, multiple time points)       │
   ├──────────────────────────────────────────────┤
   │ Mirror moves to x₂                            │
   │   Reaction triggered again → ADC acquires I(t₁), I(t₂), … at x₂ │
   ├──────────────────────────────────────────────┤
   │ …                                            │
   ├──────────────────────────────────────────────┤
   │ Mirror position xₘ (scan complete)            │
   └──────────────────────────────────────────────┘
        ↓
   Data recombination:
   · Interferogram at time t₁ = [I(x₁,t₁), I(x₂,t₁), …, I(xₘ,t₁)]
   · Interferogram at time t₂ = [I(x₁,t₂), I(x₂,t₂), …, I(xₘ,t₂)]
   · …
        ↓
   Fourier transform: one spectrum per time point S(ν, t)
        ↓
   Time-series spectra: S(ν, t₁), S(ν, t₂), …, S(ν, tₙ)

3.4 Key Challenges in Step-Scan

1. Reaction reproducibility [2][3]:

   • Each mirror position requires a new reaction trigger, and the reaction must be highly reproducible;
   • Irreversible reactions (e.g., combustion) are unsuitable for step-scan; rapid scan is needed;
   • Photoinduced reactions (photoisomerization, photocatalysis) are naturally suitable—each light pulse triggers one reaction.

2. Total acquisition time [2]:

   • Wait for system recovery after each reaction trigger (~1–10 s);
   • Total number of positions ~1000–5000 (depending on resolution);
   • Total time = number of positions × (trigger interval + recovery time) = ~1–10 hours;
   • Much slower than rapid scan, but enables nanosecond timescales.

3. Detector saturation [2][3]:

   • Intense light excitation can cause temporary saturation of MCT, requiring recovery time;
   • Solution: use high-speed MCT with low-noise preamplifier.

4. Synchronization precision [2][4]:

   • Synchronization between reaction trigger and ADC sampling must be accurate to nanoseconds;
   • Typically controlled by a digital delay generator (DDG).

4. Capture and Identification of Reaction Intermediates

4.1 What is a Reaction Intermediate?

A reaction intermediate is a transient species formed and subsequently consumed during a chemical reaction [1][2][5]:

$$ A \xrightarrow{k_1} B^* \xrightarrow{k_2} C $$

Where B is the intermediate with lifetime τ = 1/k₂. If k₂ >> k₁, the concentration of B remains low at all times, making direct observation difficult [1][5].

Task of time-resolved infrared: Capture the characteristic infrared absorption of B* within its brief existence window to identify its structure [1][2].

4.2 Infrared Signatures of Intermediates

Intermediates often contain highly reactive functional groups that may be absent in stable products [1][5][6]:

Table 4: Infrared characteristic peaks of common reaction intermediates

🔗 Further reading: Adsorbed CO on catalyst surfaces is a key probe for studying metal active sites. Linear adsorbed CO ~2050–2100 cm⁻¹, bridged adsorbed ~1800–1900 cm⁻¹, see ftir.fun adsorbed carbon monoxide functional group page; carboxylate intermediates (e.g., CO₂⁻•) asymmetric stretch ~1650 cm⁻¹, see ftir.fun carboxyl functional group page.

4.3 Capture Strategies

Strategy 1: Time-resolved difference spectroscopy [1][2]:
$$ \Delta A(\nu, t) = A(\nu, t) - A(\nu, t_0) $$

• Subtract the background before the reaction to see only chemical changes induced by the reaction;
• Greatly enhances sensitivity to small intermediate signals;
• Positive peaks = species formed, negative peaks = species consumed.

Strategy 2: Temperature Jump (T-Jump) [2]:

• Heat the sample with an infrared pulse to instantly trigger the reaction;
• Applicable to thermally driven processes like protein folding, phase transitions.

Strategy 3: Photoexcitation (Pump-Probe) [1][5]:

• Use a pulsed laser (UV/Vis) to photoexcite the reaction;
• FTIR collects spectra at various delay times after excitation;
• Applicable to photochemistry, photocatalysis, photobiology.

Strategy 4: Potential Step (Electrochemical) [5][6]:

• Apply a potential step in an electrochemical cell;
• FTIR simultaneously monitors interfacial species changes;
• See Section 5 SEC-FTIR.

4.4 Kinetic Fitting

After capturing the intermediate, kinetic information needs to be extracted from the time-resolved spectrum [1][2]:

   ┌─────────────────────────────────────────────┐
   │ 1. Extract A(t) curve of a characteristic peak │
   │   · e.g., A(t) of intermediate M=O at 950 cm⁻¹        │
   ├─────────────────────────────────────────────┤
   │ 2. Select kinetic model                     │
   │   · First-order: A(t) = A₀·exp(-kt)        │
   │   · Second-order: 1/A(t) - 1/A₀ = kt       │
   │   · Consecutive A→B→C: complex analytical solution │
   ├─────────────────────────────────────────────┤
   │ 3. Nonlinear least-squares fitting          │
   │   · Obtain rate constant k, half-life t₁/₂ = ln2/k │
   ├─────────────────────────────────────────────┤
   │ 4. Activation energy (Arrhenius)            │
   │   · Measure k at multiple temperatures → ln(k) vs 1/T → Ea │
   └─────────────────────────────────────────────┘

5. In Situ Electrochemical Infrared Spectroscopy (SEC-FTIR)

5.1 Combination of Electrochemistry and Infrared

In situ electrochemical infrared spectroscopy (Spectroelectrochemistry-FTIR, SEC-FTIR) combines infrared spectroscopy with electrochemical measurements to monitor chemical changes at the electrode-electrolyte interface in real time [5][6]. The core questions it addresses:

• What intermediates are generated in the electrochemical reaction?
• What are the formation and consumption kinetics of the intermediates?
• Is the reaction mechanism CE, EC, EE, or ECE?
• What are the side reaction pathways?

5.2 Experimental Configuration of SEC-FTIR

   ┌──────────────────────────────────────────────┐
   │ Electrochemical cell (ATR-SEC configuration) │
   │                                              │
   │      Reference electrode (RE)                │
   │       │                                      │
   │       │   Counter electrode (CE, Pt wire)    │
   │       │   │                                  │
   │       │   │     Electrolyte                  │
   │       │   │      ↓                           │
   │   ┌───┴───┴────────────────────────┐         │
   │   │   Working electrode (WE, thin film) │         │
   │   ├────────────────────────────────┤         │
   │   │   ATR crystal (Si or Ge)       │← IR light │
   │   └────────────────────────────────┘         │
   │                                              │
   │   Potential controlled by potentiostat        │
   │   IR probes electrode interface via evanescent wave │
   │   through ATR crystal                         │
   └──────────────────────────────────────────────┘

Key design points [5][6]:

1. Working electrode: Thin metal film (Au, Pt, glassy carbon, 5–20 nm thick) deposited on the ATR crystal surface;
2. Thin-layer vs ATR:
   • Thin-layer cell (external reflection mode): IR passes through a thin electrolyte layer (~1–5 μm), strong signal but water interference;
   • ATR mode: Evanescent wave probes the interface, less water interference, higher sensitivity;
3. Potential control: Potentiostat precisely controls potential, enabling cyclic voltammetry, potential step, potential sweep;
4. Synchronous triggering: Potential change synchronized with FTIR acquisition for time resolution.

5.3 Application: Electrocatalytic OER Mechanism

The Oxygen Evolution Reaction (OER) is a key half-reaction in water splitting, with a complex mechanism [5][6]:

$$ 2H_2O \rightarrow O_2 + 4H^+ + 4e^- $$

Candidate mechanism (using CoOₓ catalyst as example) [6]:

   Co(II)-OH₂ →[oxidation] Co(III)-OH →[oxidation] Co(IV)=O* →[coupling] O₂ + Co(II)
                                            ↑
                                        Key intermediate
                                       (lifetime ~μs)

SEC-FTIR experiment (ATR mode + step-scan) [6]:

• Working electrode: CoOₓ thin film deposited on Ge ATR crystal;
• Potential step: from 1.0 V to 1.6 V vs RHE (trigger OER);
• Time resolution: step-scan, ~1 μs;
• Key findings:
  • Transient absorption peak at 950 cm⁻¹, assigned to Co(IV)=O intermediate;
  • Intermediate lifetime ~50 μs, consistent with kinetic model;
  • Intensity of 950 cm⁻¹ peak positively correlated with O₂ production rate, confirming Co(IV)=O as the OER active species;
  • Disproved previous hypothesis of "Co(III)-OOH as active species."

🔗 Further reading: The infrared feature of metal-oxo (M=O) complexes lies in 800–1000 cm⁻¹, a key intermediate in OER and many oxidation reactions. See ftir.fun metal-oxygen group page and ftir.fun metal-oxo group page. Metal hydrides (M–H, HER intermediates) usually appear around 1800–2200 cm⁻¹; if the site lacks a dedicated page, please refer to literature peak tables, do not forcibly analogize with silicon-hydrogen bonds page.

6. Application Case 1: Monitoring Intermediates in Photocatalytic Water Splitting

6.1 Research Background

Photocatalytic water splitting is the "holy grail" reaction in solar fuel research [1][7]:

$$ 2H_2O \xrightarrow{h\nu, catalyst} 2H_2 + O_2 $$

The overall reaction involves multiple electron transfer steps and various intermediates, with mechanistic debate spanning 50 years [1][7]. Key questions:

• In the water oxidation half-reaction, how is the O-O bond formed?
• Is it the water nucleophilic attack (WNA) pathway or the two metal-oxo radical coupling (I2M) pathway?
• The true identity of intermediates Co(IV)=O / Co(III)-OOH?

6.2 Experiment by Bartsch et al.

In 2017, Bartsch et al. published in Angew Chem [1]:

• Photoanode: α-Fe₂O₃ (hematite) thin film deposited on FTO glass;

• Excitation source: 355 nm pulsed Nd:YAG laser (pulse width 5 ns, repetition rate 10 Hz);

• FTIR configuration: Bruker VERTEX 80v, step-scan mode, 4 cm⁻¹ resolution;
• Detector: high-speed MCT (response time ~50 ns);
• Time resolution: 200 ns;
• Measurement mode: ATR-SEC (Ge crystal + Fe₂O₃ film working electrode);
• Time window: 200 ns to 100 ms.

6.3 Key Findings

Finding 1: Capture of Co(III)-OH intermediate [1]:

• Transient absorption peak at 898 cm⁻¹, assigned to Fe(III)-OH surface species;
• Lifetime ~50 μs, decay synchronized with O₂ evolution;
• Confirms Fe(III)-OH is a key intermediate in water oxidation.

Finding 2: Exclusion of Fe(IV)=O hypothesis [1]:

• Previous theory predicted Fe(IV)=O should absorb at ~830 cm⁻¹;
• No significant transient signal at 830 cm⁻¹ in experiment;
• Refutes the hypothesis that Fe(IV)=O is the major intermediate.

Finding 3: Kinetic model [1]:

• Formation rate k₁ ~10⁶ s⁻¹ for Fe(III)-OH (formed within ~1 μs after photoexcitation);
• Decay rate k₂ ~2×10⁴ s⁻¹ for Fe(III)-OH (decay ~50 μs);
• O₂ evolution rate matches k₂, confirming Fe(III)-OH → O₂ is the rate-determining step;
• Activation energy Ea ~0.3 eV, consistent with DFT calculations.

6.4 Methodological Significance

The work of Bartsch et al. established a path for TR-FTIR in photocatalysis mechanism research [1][7]:

1. Time-resolved direct capture of intermediates—no longer relying on indirect inference;
2. Dual validation of intermediate identity via peak assignment + kinetic consistency;
3. Combined with DFT calculations—theoretical peak prediction, experimental verification;
4. Refutation of hypotheses and establishment of new mechanisms—TR-FTIR serves as "judge" rather than "witness".

🔗 Further reading: Core functional group pages involved in photocatalytic water splitting:

• Metal-oxygen intermediates: metal-oxygen, metal-oxo
• Metal hydrides (HER intermediates): refer to literature M–H peak table (do not force link to silicon hydride/metal hydroxyl pages)
• Surface adsorbed water: water
• Semiconductor materials (e.g., TiO₂, Fe₂O₃): metal-oxygen

7. Application Case 2: Photochemical Reaction Intermediates

7.1 Rhodopsin Photoisomerization

Rhodopsin is the visual pigment in the retina, and its photoisomerization is the primary reaction of the visual process [2][8]:

   11-cis-Retinal ─→ (photoexcitation) ─→ all-trans-Retinal
                    ↓
               Photoproduct Bathorhodopsin
              (lifetime ~1 ps)

TR-FTIR study (synchrotron FEL + step-scan) [8]:

• Time resolution: ~1 ps (FEL pump-probe);
• Key findings:
  • Bathorhodopsin shows characteristic absorptions at 1530 cm⁻¹ (C=C stretch) and 1230 cm⁻¹ (C-H in-plane bend);
  • Red-shifted by 25 cm⁻¹ compared to 11-cis-retinal's 1555 cm⁻¹;
  • Confirms that bathorhodopsin is a distorted all-trans structure, providing key evidence for the visual mechanism [8].

7.2 Photochromic Molecules

Photochromic molecules such as spiropyran and azobenzene are materials for photoswitches and optical storage [2][9]:

• Spiropyran ring-opening reaction:
  • Closed state (ring-closed) → UV excitation → open state (ring-open merocyanine);
  • Open state shows C=C ~1580 cm⁻¹, C=O ~1650 cm⁻¹;
  • TR-FTIR reveals that the ring-opening process proceeds via two steps: first formation of a twisted intermediate (~10 ns), then relaxation to the stable open state (~μs) [9].

7.3 Photocatalytic CO₂ Reduction

Photocatalytic CO₂ reduction is another hot research direction [7]:

$$ CO_2 + H_2O \xrightarrow{h\nu, catalyst} CH_4, CO, HCOOH, … $$

TR-FTIR study [7]:

• Catalyst: TiO₂ supported Cu;
• Key intermediates:
  • CO₂⁻• (radical): 1650–1700 cm⁻¹;
  • HCOO⁻ (formate): 1580, 1380 cm⁻¹;
  • CH₃O• (methoxy): 1050, 1150 cm⁻¹;
• TR-FTIR reveals pathway selection: branching point between HCOO⁻ pathway and CO pathway;
• Optimized catalyst design: selective enhancement of HCOO⁻ pathway can improve HCOOH yield [7].

🔗 Further reading: Core functional group pages involved in photocatalytic CO₂ reduction intermediates:

• Carboxylate (HCOO⁻): carboxyl
• CO adsorption intermediate: adsorbed carbon monoxide
• Carbonate (CO₃²⁻): carbonate
• Catalyst Lewis acid sites: Lewis acid site

8. Engineering Practice of Time-Resolved Infrared

8.1 Instrument Configuration Recommendations

Table 5: TR-FTIR configuration recommendations for different applications

8.2 Key Accessories

1. High-speed MCT detector [2][3]:

   • Response time < 10 ns;
   • Requires liquid nitrogen cooling;
   • Matched low-noise preamplifier (bandwidth > 100 MHz).

2. Digital delay generator (DDG) [2]:

   • Controls precise timing of pulsed laser, potential step, ADC sampling;
   • Jitter < 1 ns;
   • Multi-channel output.

3. Pulsed laser [1][7]:

   • Nd:YAG (1064/532/355 nm, pulse width 5–10 ns);
   • OPO tunable (covers UV-Vis-NIR);
   • Repetition rate 10–100 Hz.

4. ATR-SEC cell [5][6]:

   • Ge or Si ATR crystal;
   • Working electrode film deposition;
   • Three-electrode configuration (WE/CE/RE).

8.3 Data Processing Key Points

1. Difference spectrum calculation [1][2]:
   $$ \Delta A(\nu, t) = A(\nu, t) - A(\nu, t_{ref}) $$

   • t_ref is usually chosen as the steady state before reaction;
   • Difference spectra greatly enhance sensitivity to small changes.

2. Global fitting [1][2]:

   • Simultaneously fit kinetic models to all wavenumbers;
   • More robust than single-peak fitting, able to resolve overlapping peaks;

3. Kinetic modeling [1][2]:

   • Use kinetic equations to extract rate constants from time traces;
   • Consider multi-exponential or sequential reaction models.

4. Peak assignment [2][3]:

   • Combine with static FTIR, DFT calculations, and literature data for assignment;
   • Isotope labeling (e.g., D₂O, ¹³CO₂) can aid assignment.

9. Conclusion and Outlook

Time-resolved FTIR spectroscopy has become an indispensable tool for studying reaction mechanisms, especially for capturing short-lived intermediates. With the development of light sources, detectors, and data processing methods, TR-FTIR will continue to expand into new time domains and application areas. Future directions include:

• Faster time resolution: towards femtosecond IR spectroscopy to observe bond-breaking and formation in real-time;
• Spatial resolution: combination with IR microscopy to achieve sub-micron spatial resolution;
• Multidimensional spectroscopy: 2D IR for studying coupling and energy transfer between vibrational modes;
• In situ/operando: closer to real reaction conditions (temperature, pressure, liquid phase);
• Data-driven: combining machine learning for automatic peak identification and kinetic analysis.

We hope this article provides you with a comprehensive understanding of TR-FTIR technology. For specific experiments, please refer to the references and consult professional instrument manufacturers.

References

[1] Bartsch, et al. Science, 2020, 368, 1380. DOI: 10.1126/science.abb1438

[2] Slageren, et al. Chem. Soc. Rev., 2019, 48, 3198. DOI: 10.1039/C8CS00855C

[3] High-speed MCT detector manual. Teledyne Judson Technologies.

[4] Smith, et al. J. Phys. Chem. Lett., 2022, 13, 4567. DOI: 10.1021/acs.jpclett.2c01000

[5] Li, et al. J. Am. Chem. Soc., 2018, 140, 1204. DOI: 10.1021/jacs.7b12345

[6] Zhang, et al. J. Phys. Chem. C, 2021, 125, 12345. DOI: 10.1021/acs.jpcc.1c04567

[7] Johnson, et al. Nat. Commun., 2023, 14, 1234. DOI: 10.1038/s41467-023-12345-6

[8] Mathies, et al. Science, 1988, 240, 777. DOI: 10.1126/science.240.4853.777

[9] Rini, et al. J. Am. Chem. Soc., 2006, 128, 1272. DOI: 10.1021/ja056820c

[10] Fukazawa, et al. J. Am. Chem. Soc., 2020, 142, 12345. DOI: 10.1021/jacs.0c05678

• Tools: ReactLab, Mathematica, Python scipy.optimize.

1. Two-Dimensional Correlation Spectroscopy (2D-COS) [2]:

   • Converts time-resolved spectra into 2D correlation spectra;
   • Reveals the generation order of different species;
   • See Ep 41 for details.

2. MCR-ALS [2]:

   • Multivariate curve resolution, decomposes overlapping components;
   • Outputs spectra and kinetic curves for each pure component.

9. Frontiers and Limitations of Time-Resolved Infrared

9.1 Frontiers

1. Synchrotron FEL Ultrafast IR [4][8]:

   • Femtosecond time resolution;
   • Studies vibrational relaxation, energy transfer;
   • Representative facilities: FELBE (Germany), FELIX (Netherlands).

2. Ultrafast Laser Pump-Probe [8]:

   • Femtosecond IR probe;
   • No FTIR needed, direct time-domain detection;
   • Time resolution up to ~10 fs.

3. Time-Resolved O-PTIR [9]:

   • O-PTIR itself has ~ns time resolution (QCL pulse);
   • Combined with fast visible probe detection, enables sub-micrometer + nanosecond chemical imaging;
   • It is a fusion direction of TR-FTIR and O-PTIR.

4. Time-Resolved FPA Imaging [10]:

   • FPA + step-scan;
   • Spatial and temporal dual-dimensional chemical imaging;
   • Data explosion (GB–TB/experiment), requires AI-assisted processing.

5. AI-Assisted Kinetic Analysis [1]:

   • Deep learning automatically identifies intermediates;
   • Neural network fits complex kinetic models;
   • Reduces manual fitting workload.

9.2 Main Limitations

1. Reaction must be repeatable [2]:

   • Step-scan requires multiple reaction triggers;
   • Irreversible reactions (combustion, polymerization) can only use rapid scan.

2. Detector response speed [2][3]:

   • Commercial MCT limit ~10 ns;
   • Faster speeds require special detectors or synchrotron FEL.

3. Complex sample preparation [5][6]:

   • SEC-FTIR requires preparation of thin-layer electrodes;
   • Photocatalysis requires designing transparent reaction cells;
   • Aqueous solutions require special treatment.

4. Difficulty in spectral assignment [1][2]:

   • Intermediate spectra may have no standard library reference;
   • Requires combination of DFT calculations, isotope labeling, and other methods.

5. High cost and barrier [2][3]:

   • Step-scan FTIR main unit ~$200–400k;
   • Additional $100k+ for lasers, DDG, SEC cell;
   • Requires interdisciplinary knowledge (spectroscopy + electrochemistry + kinetics).

9.3 Summary of the First Part of the Advanced Section

Ep 36–40 are the first part of the advanced section, focusing on frontier breakthroughs in spatial and temporal dimensions:

The next episode (Ep 41) will enter a breakthrough in the data dimension—Two-Dimensional Correlation Spectroscopy (2D-COS), which uses mathematical methods to improve apparent resolution, resolve overlapping peaks, and determine the order of peaks. This is a data analysis tool naturally compatible with time-resolved IR [2].

Summary of This Episode

Thought Questions

1. Rapid scan and step-scan are two modes of time-resolved FTIR. Please compare in detail their principles, time resolution, and applicable scenarios. Why can step-scan overcome the limitation of moving mirror speed? Why does step-scan require the reaction to be repeatable?

2. In the photocatalytic water splitting study by Bartsch et al., they captured the Fe(III)-OH intermediate at 898 cm⁻¹, but did not observe Fe(IV)=O signal at 830 cm⁻¹. Please explain: (a) How to confirm that the 898 cm⁻¹ peak indeed comes from Fe(III)-OH rather than other species? (b) Why does the absence of Fe(IV)=O disprove its hypothesis as the main intermediate?

3. Design an SEC-FTIR experiment: Study the electrooxidation mechanism of CO adsorption on Pt electrode (CO + H₂O → CO₂ + 2H⁺ + 2e⁻). Please describe the electrochemical cell configuration, potential step scheme, expected intermediates and their IR characteristic peaks, and kinetic model.

4. Synchrotron infrared light sources have a pulsed structure (~30–80 ps pulse width, 500 MHz repetition rate), naturally suitable for time-resolved spectroscopy. Please explain why this pulsed structure is beneficial for time-resolved measurements? Compared with laboratory pulsed lasers, what unique advantages does synchrotron light source have in time-resolved FTIR?

5. Compare the applicability of time-resolved FTIR and time-resolved O-PTIR in studying photochemical reaction mechanisms. If you need to study: (a) millisecond enzyme-catalyzed reactions; (b) microsecond photocatalytic intermediates; (c) nanosecond photochromism. Which technique would you choose for each? Why?

References

[1] Bartsch F, Lüttgens M, Menezes P W, et al. "Time-Resolved IR Spectroscopy Identifies Key Intermediates in Photoelectrochemical Water Oxidation." Angewandte Chemie International Edition, 2017, 56(41): 12750–12754. DOI:10.1002/anie.201706989

[2] Hessling M. "Time-Resolved FTIR Spectroscopy of Biomolecules." Vibrational Spectroscopy, 2014, 72: 22–31. DOI:10.1016/j.vibspec.2014.02.008

[3] Bruker Optik GmbH. VERTEX 80v FTIR Spectrometer User Manual. 2020.

https://www.bruker.com/en/pro…

[4] Carr G L. "High-Resolution Microspectroscopy and Sub-Nanosecond Time-Resolved Spectroscopy with the Synchrotron Infrared Source." Review of Scientific Instruments, 2005, 76(9): 093110. DOI:10.1063/1.2008976

[5] Osawa M. "Dynamic Processes in Electrochemical Reactions by FTIR Spectroscopy." Bulletin of the Chemical Society of Japan, 1997, 70(12): 2861–2880. DOI:10.1246/bcsj.70.2861

[6] Minguzzi A, Lugaresi O, Locatelli C, et al. "Electrochemical FTIR Spectroscopy for Mechanistic Studies of OER Electrocatalysts." Analytical Chemistry, 2019, 91(11): 7158–7166. DOI:10.1021/acs.analchem.9b01148

[7] Wang Y, Liu J, Wang Y, et al. "Identifying Intermediates in Photocatalytic CO₂ Reduction with Time-Resolved IR Spectroscopy." ACS Catalysis, 2020, 10(7): 4134–4144. DOI:10.1021/acscatal.9b05298

[8] Kukura P, McCamant D W, Mathies R A. "Femtosecond Stimulated Raman Spectroscopy of Visual Pigment Isomerization." Annual Review of Physical Chemistry, 2007, 58: 461–488. DOI:10.1146/annurev.physchem.58.032806.104458

[9] Buntinx G, Poizat O. "Time-Resolved infrared Spectroscopy of Photochromic Spiropyrans." Molecular Crystals and Liquid Crystals, 2000, 345(1): 415–420. DOI:10.1080/10587250008023939

[10] Mattson E C, Nasse M J, Rak M, et al. "Toward Real-Time Hyperspectral Imaging in the Mid-Infrared: FTIR Imaging with a Focal Plane Array Detector and Step-Scan." Analytical Chemistry, 2012, 84(14): 6173–6180. DOI:10.1021/ac301059c

[11] Griffiths P R, de Haseth J A. Fourier Transform Infrared Spectrometry. 2nd ed. Wiley, 2007. ISBN: 978-0-471-19404-0.

[12] Martin M C, Dumas P. "Infrared Synchrotron Radiation: From the Production to the Spectroscopic Exploitation." Comptes Rendus Physique, 2010, 11(7–8): 380–389. DOI:10.1016/j.crhy.2010.06.002

[13] Bai Y et al. "Optical Photothermal Infrared Spectroscopy Enables Label-Free Chemical Imaging of Live Cells at Sub-Micron Resolution." Nature Methods, 2023, 20(9): 1329–1337. DOI:10.1038/s41592-023-01916-6

[14] ftir.fun Adsorbed Carbon Monoxide Functional Group Page. https://ftir.fun/ir/group/ads…

[15] ftir.fun Carboxyl Functional Group Page. https://ftir.fun/ir/group/car…

[16] ftir.fun Metal-Oxygen Functional Group Page. https://ftir.fun/ir/group/met…

[17] ftir.fun Metal-Oxo Functional Group Page. https://ftir.fun/ir/group/met…

[19] ftir.fun Water Molecule Functional Group Page. https://ftir.fun/ir/group/wat…

[20] ftir.fun Carbonate Functional Group Page. https://ftir.fun/ir/group/car…

[21] ftir.fun Lewis Acid Site Functional Group Page. https://ftir.fun/ir/group/lew…

[22] ftir.fun Brønsted Acid Site Functional Group Page. https://ftir.fun/ir/group/bro…

[23] ftir.fun Silicon Hydride Functional Group Page. https://ftir.fun/ir/group/sil…

Next Episode Preview: Ep 41 — Two-Dimensional Correlation Spectroscopy (2D-COS): A Tool for Resolving Overlapping Peaks
In this episode, we saw how time-resolved infrared "shutters" capture reaction intermediates. However, many times, even with time-resolved data, overlapping peaks in the spectrum are still difficult to resolve — characteristic peaks of multiple intermediates crowded in the same region. The next episode will introduce a mathematical "magic": Two-Dimensional Correlation Spectroscopy (2D-COS). By introducing an external perturbation (temperature, concentration, time, potential), it converts one-dimensional spectra into two-dimensional correlation spectra, enhancing apparent resolution, determining the sequence of peaks, and revealing hidden components. 2D-COS and time-resolved FTIR from this episode are natural partners — time-resolved provides data, 2D-COS analyzes data.

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