Photodissociation of gaseous CH3COSH at 248 nm by time-resolved Fourier-transform infrared emission spectroscopy: Observation of three dissociation channels

Abstract

Upon one-photon excitation at 248 nm, gaseous CH(3)C(O)SH is dissociated following three pathways with the products of (1) OCS + CH(4), (2) CH(3)SH + CO, and (3) CH(2)CO + H(2)S that are detected using time-resolved Fourier-transform infrared emission spectroscopy. The excited state (1)(n(O), π*(CO)) has a radiative lifetime of 249 ± 11 ns long enough to allow for Ar collisions that induce internal conversion and enhance the fragment yields. The rate constant of collision-induced internal conversion is estimated to be 1.1 × 10(-10) cm(3) molecule(-1) s(-1). Among the primary dissociation products, a fraction of the CH(2)CO moiety may undergo further decomposition to CH(2) + CO, of which CH(2) is confirmed by reaction with O(2) producing CO(2), CO, OH, and H(2)CO. Such a secondary decomposition was not observed previously in the Ar matrix-isolated experiments. The high-resolution spectra of CO are analyzed to determine the ro-vibrational energy deposition of 8.7 ± 0.7 kcal/mol, while the remaining primary products with smaller rotational constants are recognized but cannot be spectrally resolved. The CO fragment detected is mainly ascribed to the primary production. A prior distribution method is applied to predict the vibrational distribution of CO that is consistent with the experimental findings.

Collisional Energy Transfer from Vibrationally Excited Hydrogen Isocyanide

“…In our studies, time-resolved Fourier transform infrared (FTIR) emission spectroscopy was used to experimentally interrogate the photofragments generated following photolysis of a given precursor cyanide. Time-resolved FTIR emission, demonstrated on a nanosecond time scale by our laboratory, provides a sensitive measure of a molecule’s time-dependent internal energy and has previously been used to characterize photodissociation dynamics, − the spectroscopy of radicals and transients, − and energy transfer kinetics of highly excited molecules. − In general, time-resolved FTIR emission is capable of simultaneously monitoring emission from all vibrationally excited reaction channel products. Any fragment that is produced vibrationally cold, however, will not produce IR emission and therefore cannot be detected with this method.…”

Photodissociation of Propionaldehyde at 248 nm: Roaming Pathway as an Increasingly Important Role in Large Aliphatic Aldehydes

“…In the experiments with the step-scan FTIR emission spectroscopy, , the photolysis laser emitting at 248 nm propagated through the sample beam ejected at a pressure of ∼0.5–1 Torr from an effusive nozzle that was housed in a reaction chamber. The fragment emission signals were guided to the entrance of the FTIR spectrometer in which the movable mirror of the interferometer was allowed to move step-by-step.…”

“…Upon irradiation of C 2 H 5 CHO at 248 nm in the presence of Ar at 3 Torr, a series of 5 μs resolved spectra of CO that appears at 1900–2200 cm –1 is well-resolved with a 0.25 cm –1 resolution up to J = 46, 45, 42, and 37 for ν = 1, 2, 3, and 4, respectively. (Figure S1 in the Supporting Information) The addition of Ar facilitates the IC efficiency with which the CO and C 2 H 6 products are associated,and simultaneously quenches the rovibrational distributions of the resultant products if the delay time is long enough. , …”

“…The rotational energy E rot (ν) at a given ν may be evaluated by E rot ( ν ) = prefix∑ J = 1 J = max P ν , J E ν , J where P ν, J is the fraction of molecules populated at the J level. Given the above vibrational population ratio, the CO rotational energy disposal is averaged to be 2.1 ± 0.1 kcal/mol. , Further, the Boltzmann plot of ln( P ν ) versus E ν / k B at 0 μs delay yields a straight line, from which the vibrational temperature is determined to be 5200 ± 100 K (Figure S5 in Supporting Information). Here, P ν denotes the fraction of vibrational population, E ν is the vibrational energy, and k B is the Boltzmann constant.…”

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193 nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

“…Time-resolved FTIR emission spectroscopy [51][52][53][54][55] with some modification was employed in this work, as illustrated in Fig. S5.…”