IR lasers

Quack M 1991 Mode selective vibrational redistribution and unimolecular reactions during and after IR-laser excitation Mode Selective Chemistry ed J Jortner, R D Levine and B Pullman (Dordrecht Kluwer) pp 47-65... 

Chapman W B, Blackman B W, Nizkorodov S and Nesbitt D J 1998 Quantum-state resolved reactive scattering of F + H2 in supersonic jets Nascent HF(v,J) rovibrational distributions via IR laser direct absorption methods J. Chem. Rhys. 109 9306-17... 

This technique with very high frequency resolution was used to study the population of different hyperfme structure levels of the iodine atom produced by the IR-laser-flash photolysis of organic iodides tluough multiphoton excitation ... 

A) During the luultiphoton excitation of molecular vibrations witli IR lasers, many (typically 10-50) photons are absorbed in a quasi-resonant stepwise process until the absorbed energy is suflFicient to initiate a unimolecular reaction, dissociation, or isomerization, usually in the electronic ground state. 

The record m the number of absorbed photons (about 500 photons of a CO2 laser) was reached with the CgQ molecule [77]. This case proved an exception in that the primary reaction was ionization. The IR multiphoton excitation is the starting pomt for a new gas-phase photochemistry, IR laser chemistry, which encompasses numerous chemical processes. 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

Table B2.5.3. Product energy distribution for some IR laser chemical reactions. (E ) is the average relative translational energy of fragments, is the average vibrational and rotational energy of polyatomic fragments, and/ is the fraction of the total product energy appearing as translational energy [109],...

Figure B2.5.14. The IR laser chemistry of CF I excited up to the dissociation energy with about 17 quanta of a CO2 laser, The dissociation is detected by uncertainty limited cw absorption (hv ), see figures...

Figure B2.5.15. Iodine atom fonnation in the IR laser ehemistry of CF I (exeitation at 1074.65 em probe on the F = 4 —> F = 3 hyperfme stnieture transition, see figure B2.5.12.) (a) The absorbanee as a fiinetion of time (effeetive absorption eross seetion frill eiirve, left ordinate) shows elear steps at eaeh maximum of the mode loeked CO2 laser pulse sequenee (intensity, broken eurve, right ordinate), (b) The Ifaetion Fp of dissoeiating moleeules as a frinetion of fluenee F.

In exeeptional oases, the IR laser exeitation oan lead to ionization. An interesting example is die C02-laser-induoed ionization of C q, where n > 500 photons are absorbed and vibrations are exoited far beyond the... 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Figure B2.5.17. (a) Time-dependent intensity / and redueed fluenee F/Fq for a single-mode CO2 laser pulse used in the IR laser photolysis of CF I. Fq is the total fluenee of the laser pulse, (b) VIS-REMPI iodine atom signals obtained with CO2 laser pulses of different fluenee (after [113]).

Apart from the obvious property of defining pulses within short time intervals, the pulsed laser radiation used in reaetion kineties studies ean have additional partieular properties (i) high mtensity, (ii) high monoehromatieity, and (iii) eoherenee. Depending on the type of laser, these properties may be more or less pronouneed. For instanee, the pulsed CO2 lasers used in IR laser ehemistry easily reaeh intensities between... 

Quack M 1995 IR laser chemistry Infrared Rhys. Technol. 36 365-80... 

Quack M and Thdne H J 1987 Absolute and relative rate coefficients in the IR-laser chemistry of bichromophoric fluorobutanes tests for inter- and intra-molecular selectivity Chem. Rhys. Lett. 135 487-94... 

The vibrations of molecular bonds provide insight into bonding and stmcture. This information can be obtained by infrared spectroscopy (IRS), laser Raman spectroscopy, or electron energy loss spectroscopy (EELS). IRS and EELS have provided a wealth of data about the stmcture of catalysts and the bonding of adsorbates. IRS has also been used under reaction conditions to follow the dynamics of adsorbed reactants, intermediates, and products. Raman spectroscopy has provided exciting information about the precursors involved in the synthesis of catalysts and the stmcture of adsorbates present on catalyst and electrode surfaces. 

Vj = 1 <— v" = 1 transition will be at a different energy than the Vj = 0 <— v" = 0. We use this fact to measure the vibrational spectrum of V (OCO) in a depletion experiment (Fig. 12a). A visible laser is set to the Vj = 0 Vj = 0 transition at 15,801 cm producing fragment ions. A tunable IR laser fires before the visible laser. Absorption of IR photons removes population from the ground state, which is observed as a decrease in the fragment ion signal. This technique is a variation of ion-dip spectroscopy, in which ions produced by 1 + 1 REMPI are monitored as an IR laser is tuned. Ion-dip spectroscopy has been used by several groups to study vibrations of neutral clusters and biomolecules [157-162]. 

Figure 12. Vibrational action spectra of V (OCO) in the OCO antisymmetric stretch region, (a) Spectrum obtained by monitoring depletion in the photofragment produced by irradiation at the vibronic origin at 15,801 cm The IR absorption near 2391.5 cm removes molecules from V[" = 0, leading to an 8% reduction in the fragment yield, (b) Spectrum obtained by monitoring enhancement in the VO+ photofragment signal as the IR laser is tuned, with the visible laser fixed at 15,777 cm (the Vj = 1 v" = 1 transition). The simulated spectrum gives a more precise value of the OCO antisymmetric stretch vibration in V" (OCO) of 2392.0 cm .

The use of near-IR-laser excited FT-SERS eliminates the disturbing fluorescence of impurities found with visible excitation, and provides SERS enhancement factors that are about 20 times larger than those found for excitation at 514.5nm [792]. For a strong Raman scatterer (fluorene), a typical detection limit of 500 ng is found for a 3-mm diameter spot. For weak scatterers, the detection limits may be in the high- xg region, which means that some compromise between chromatographic... 

In both cases, GC fingerprint libraries must be built before quantitative analysis can be routinely carried out. In analysis of QTLC by laser pyrolysis scanning (LPS), the TLC plates are placed in a chamber after development, and were irradiated with an IR laser to produce a high temperature at the location of the spot. The analyte is swept by a carrier gas to a GC, and detected with FID or ECD. The technique combines the separation power of TLC and the detection modes of GC [846]. 

Visible lasers are typically used for sample excitation, although near-IR lasers can be used when visible excitation sources cause sample fluorescence, obscuring the Raman scatter. 

Fig. 3. Schematic diagram of the Northwestern apparatus for IR laser kinetic measurements in the gas phase. D, and D2 are InSb detectors with D2 being a high speed photovoltaic detector. M = Mirror, I = iris, C = chopper, BS = beam splitter, P = photolysis cell. [Reproduced with permission from Ouderkirk et al. (75).]...

Fig. 6. Schematic diagram of the Nottingham apparatus for IR kinetic measurements on solutions. Solid lines represent the light path, broken lines the electrical connections. L = Line tunable CO laser, S = sample cell, F = flash lamp, P = photodiode, D = fast MCT IR detector, T = transient digitizer, O = oscilloscope, and M = microcomputer. Nonfocussing optics were used throughout, and the IR laser beam was heavily attenuated by a variable path cell V, filled with liquid methanol, placed immediately in front of the detector. [Reproduced with permission from Moore et al. (61).]...

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