YUAN-PERN LEE LABORATORY

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Step-scan FTIR (Emission)

Introduction

     Experiments involving infrared (IR) product emission studies have an important role in dynamics to provide information about the rotational and vibrational distributions of reaction products. Emission has the greater sensitivity than absorption measurements, because the fluorescence signal has a small background, but emission spectra provide no information about the vibrational ground state (v = 0). Using time-resolved FTIR, we readily obtain the complete temporal evolution of the emission spectrum; analysis of the transformed spectra at varied durations of reaction allows the vibrational and rotational distribution of the photofragment to be monitored with time, and extrapolated to obtain the nascent distribution. The major advantages of FTIR spectroscopy are that all light frequencies are observed simultaneously at the detector (Felgett advantage) and the energy flux reaching the detector is large (Jacquinot advantage), and have enabled high-resolution spectra to be obtained routinely in short periods with excellent sensitivity. In our instrument (Bruker IFS 66v/s), the spectral resolution attains 0.13 cm-1 and temporal resolution attains 50 ns.

Pronciple of Step-scan FTIR

     The step-scan means that the moving mirror of the interferometer pauses at certain points, which are called zero-crossing points(ZCP), determined by the wavelength of He-Ne laser. When the moving mirror is stopped at the first zero-crossing point and stabilized, a chemical event initiated (by stimulation of some kind). During this event, the concentration of chemical substances might alter resulting in a modification of transmitted IR light. The detector receives the modulated light, and the signal (voltage or current) becomes recorded with an analog-to-digital converter. Hence, we obtain a temporal profile at this zero-crossing point. The moving mirror steps to the next ZCP and undergoes the same chemical event. The process finishes at the final required points and all temporal profiles are recorded. In an absorption experiment, we use AC/DC method to collect the data. The AC-coupled signals provide the time-resolved interferograms and the DC-coupled signal produces a spectral background and phase correction. Hence, the absorption difference ΔAt(v) can be deduced in the following form, ΔAt (v)= -log(1+ΔSt(v)/S0(v)).

  • Coutunuous Flow

     Step-scan time-resolved Fourier-transform spectroscopy(TR–FTS) is capable of quantitative measurements of IR chemiluminescence within microsecond range; the temporal evolution of populations of vibrational– rotational levels of each species detected with this technique provides much information on rates of formation and quenching. Our previous applications of this technique to photodissociation of halogen-containing compounds (ex. CH2CHF, CH2CHBr, CH2CF2, CF2CHCl, fluorobenzene and p-fluorotoluene) have been successful; observed internal state distributions of HX provide direct evidence of photoelimination of HX via three-center and four-center transition states.

     Now we report here an application of this technique to dynamics of a bimolecular reaction, such as
l + H2S → HCl (v≤2, J ≤ 12) + HS
Cl + CH3SH → HCl (v≤3, J ≤ 15) + CH3S
O(1D) + CO → O(3P)+ CO (v≤6, J ≤ 68)
O(1D) + C6H6 → C5H6+ CO (v≤6, J ≤ 33)
​
     Experimental investigations on the dynamics of bimolecular reactions or collisional quenching require special attention because these processes need collisions to facilitate reaction, but in the same way collisional quenching of the products might become significant, hence modifying the nascent rotational distribution. We have demonstrated that step-scan time-resolved Fourier-transform spectroscopy (TR-FTS) is advantageous for studying the dynamics of bimolecular reactions because its multiplex capability allows probing of all states simultaneously, hence assisting understanding the effects of rotational quenching due to collisions. Furthermore, TR-FTS has been demonstrated to be more powerful than resonance-enhanced multiphoton ionization (REMPI) or VUV LIF in detecting HX (X = F, Cl, and Br) and CO REMPI and VUV LIF detection was limited by the short lifetimes of the upper states.

Experiment

     An ArF excimer laser (Lambda Physik LPX120i), operated at 30–60 Hz with pulse energy 8–10 mJ was employed as a photolysis source. A telescope served to focus the laser beam to about 20mm2 at the reaction center with a fluence of 40– 50mJ cm-2. We estimated a photolysis yield〜 50% in the irradiated region based on an absorption cross section of different molecules. IR emission was collected with a set of Welsh mirrors and directed into the Fourier-transform spectrometer (Bruker IFS66v) through two CaF2 lenses. A CaF2 beam splitter and an InSb detector cooled to 77 K were used. The detected transient signal, amplified with a gain of 1*105 V/A (bandwidth 1.5 MHz), was further amplified with a low-noise voltage amplifier (bandwidth 1 MHz, gain typically set at 50), and sent to the internal A/D converter (16 bit, 200 kHz) of the spectrometer. In some experiments we employed the same InSb detector and amplifiers as described previously but digitized the signal with an external board (PAD1232, 40 MHz, 12 bit ADC) at 25 ns resolution.

Recent Projects

     Following collisions of O (1D) with CO, rotationally resolved emission spectra of CO (1 £ v £ 6) in the spectral region 1800-2350 cm-1 were detected with a step-scan Fourier-transform spectrometer. O (1D) was produced by photolysis of O3 with light from a KrF excimer laser at 248 nm. Upon irradiation of a flowing mixture of O3 (0.016 Torr) and CO (0.058 Torr), emission of CO (v £ 6) increases with time, reaches a maximum ~10 ms. At the earliest applicable period (2-3 ms), CO shows approximately a bimodal rotational distribution corresponding to temperatures ~8000 K and ~500 K; the proportion of these two components depends on the vibrational level. A short extrapolation from data in the period 2-6 ms leads to a nascent rotational temperature of ~10170 ± 600 K for v = 1 and ~1400 ± 40 K for v = 6, with an average rotational energy of 33 ± 6 kJ mol-1. Absorption by CO (v = 0) in the system interfered with population of low J levels of CO (v = 1). The observed vibrational distribution of (v = 2) : (v = 3) : (v = 4) : (v = 5) : (v = 6) = 1.00: 0.64 : 0.51 : 0.32 : 0.16  corresponds to a vibrational temperature of 6850 ± 750 K. An average vibrational energy of 40 ± 4 kJ mol-1 is derived based on the observed population of CO (2 £ v £ 6) and estimates of the population of CO (v = 0, 1, and 7) by extrapolation. The observed rotational distributions of CO (1£ v £ 3) are consistent with results of previous experiments and trajectory calculations; data for CO (4 £ v £ 6) are new.

Publications

  1. Photodissociation of 1,1-difluoroethene (CH2CF2) at 193 nm monitored with step-scan time-resolved Fourier-transform infrared emission spectroscopy, S.-R. Lin and Y.-P. Lee, J. Chem. Phys. 111, 9233 (1999).
  2. I. Three-center vs. four-center HCl-elimination in photolysis of vinyl chloride at 193 nm: bimodal rotational distribution of HCl (v ≤ 7) detected with time-resolved Fourier-transform spectroscopy, S.-R. Lin, S.-C. Lin, Y.-C. Lee, Y.-C. Chou, I-C. Chen, and Y.-P. Lee, J. Chem. Phys. 114, 160 (2001).
  3. Three-center versus four-center elimination in photolysis of vinyl fluoride and vinyl bromide at 193 nm: Bimodal rotational distribution of HF and HBr (v ≤ 5) detected with time-resolved Fourier-transform spectroscopy, S.-R. Lin, S.-C. Lin, Y.-C. Lee, Y.-C. Chou, I-C. Chen, and Y.-P. Lee, J. Chem. Phys. 114, 7396 (2001).
  4. Three-center vs. four-center elimination of haloethene: internal energies of HCl and HF on photolysis of CF2CHCl at 193 nm determined with time-resolved Fourier-transform spectroscopy, C.-Y. Wu, C.-Y. Chung, Y.-C. Lee, and Y.-P. Lee, J. Chem. Phys. 117, 9785 (2002).
  5. Photolysis of Oxalyl Chloride (ClCO)2 at 248 nm: Emission of CO (v’ < 3, J’ < 51) detected with time-resolved Fourier-transform spectroscopy, C.-Y. Wu, Y.-P. Lee, J. F. Ogilvie, and N. S. Wang, J. Phys. Chem. A. 107, 2389 (2003).
  6. Reaction dynamics of Cl + H2S: rotational and vibrational distribution of HCl probed with time-resolved Fourier-transform spectroscopy, K.-S. Chen, S.-S. Cheng, and Y.-P. Lee, J. Chem. Phys. 119, 4229 (2003).
  7. Reaction dynamics of Cl + CH3SH: rotational and vibrational distributions of HCl probed with time-resolved Fourier-transform spectroscopy, S.-S. Cheng, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 120, 1792 (2004).
  8. Photolysis of oxalyl chloride (ClCO)2 at 193 nm : Emission of CO(v ≤ 6, J ≤ 60) detected with time-resolved Fourier-transform spectroscopy, C.-Y. Wu, Y.-P. Lee, and N. S. Wang, J. Chem. Phys. 120, 6957 (2004).
  9. Molecular elimination in photolysis of fluorobenzene at 193 nm: internal energy of HF determined with time-resolved Fourier-transform spectroscopy, C.-Y. Wu, Y.-J. Wu, and Y.-P. Lee, J. Chem. Phys. 121, 8792, (2004).
  10. Photodissociation dynamics of formyl fluoride (HFCO) at 193 nm: Branching ratios and distributions of kinetic energy, S.-H. Lee, C.-Y. Wu, S.-K. Yang , and Y.-P. Lee, J. Chem. Phys. 123, 074326 (2005).
  • Molecular Beam

     A free molecular beam provides a collisionless condition that is suitable for studying the photodissociation dynamics. We investigated the photolysis of several species, namely vinyl chloride (CH2CHCl), fluorobenzene (C6H5F), fluorotoluene (C7H7F), and phenol (C6H5OH). Preliminary results show that nascent rotational distributions of emitting photofragments are free from rotational quenching below 100 mTorr for  a continuous flow experiment, and are comparable to those obtained in a molecular-beam experiment.

Experiment

     The supersonic jet apparatus consists of a single chamber evacuated to a base pressure 3×10-6 Torr with a 40 cm diffusion pump (1000 l/s) backed by a Dry pump. The sample gas mixed with He is expanded through a pulsed slit nozzle (General Valve, 125 mm×125 mm) located 25 mm below the photolysis and probed region. The laser crosses the molecular beam and passes through the field of view of the spectrometer. The light emitted from the photolysis region is collected with optics of Welsh type and directed to the TR-FTS with CaF2 lenses. The emitting nascent photofragments are measured during 5 ms that emitting molecules remain within the field of view of the spectrometer. A pressure in the range 0.05－0.2 mTorr is obtained with the nozzle opening for 180 – 250 ms, at a repetition rate 20－35 Hz. The opening periods were controlled in unison with a pulse generator (General Valve, IOTA).

Recent Projects

     Following photodissociation of phenol (C6H5OH) at 193 nm, rotationally resolved emission spectra of CO (1 ≤ v ≤ 4) in the spectral region 1860－2330 cm-1 were detected. After correction for rotational quenching, CO (v ≤ 3) shows a nascent rotational temperature ~4600 K.

Publications

  1. Photodissociation dynamics of vinyl chloride investigated with a pulsed slit-jet and time-resolved fourier-transform spectroscopy, M. Bahou and Y. P. Lee, Aust. J. Chem. 57, 1 (2004).