Abstract
In photodissociation of trans-formic acid (HCOOH) at 193 nm, we have observed two molecular channels of CO + H2O and CO2 + H2 by using 1 μs-resolved Fourier-transform infrared emission spectroscopy. With the aid of spectral simulation, the CO spectra are rotationally resolved for each vibrational state (v = 1–8). Each of the resulting vibrational and rotational population distributions is characteristic of two Boltzmann profiles with different temperatures, originating from either transition state pathway or OH-roaming to form the same CO + H2O products. The H2O roaming co-product is also spectrally simulated to understand the interplay with the CO product in the internal energy partitioning. Accordingly, this work has evaluated the internal energy disposal for the CO and H2O roaming products; especially the vibrational-state dependence of the roaming signature is reported for the first time. Further, given a 1 μs resolution, the temporal dependence of the CO/CO2 product ratio at v ≥ 1 rises from 3 to 10 of study, thereby characterizing the effect of conformational memory and well reconciling with the disputed results reported previously between absorption and emission methods.
Highlights
Roaming is recognized as one of the hottest topics in reaction dynamics for the past decade
Given photolysis of cis-formic acid at 193 nm, Khriachtchev et al.[35] observed a CO/CO2 product ratio of 0.4 in an Ar-matrix absorption experiment, and a ratio of 5 when trans-formic acid was substituted. Such a CO/CO2 ratio is associated with conformational memory which is used to evaluate the extent of isomerization conversion between cis- and trans-formic acid
In the quasiclassical trajectory calculations, Martınez-Nunez group[39] evaluated a CO/CO2 ratio of 22.3 and 12.2 for trans- and cis-HCOOH, respectively, corresponding to a ratio of ~2 for trans-/cis-state selected in the initial excitation, which is much smaller than the absorption result of 12.5
Summary
Roaming is recognized as one of the hottest topics in reaction dynamics for the past decade. For the most studied roaming cases of H2CO and CH3CHO1–11, following photodissociation a recoiling atom or radical fragment on the ground-state surface can roam and abstract intramolecularly another H atom to generate the H2 + CO and CH4 + CO molecular products. As an example using carbonyl compounds in the photodissociation, aliphatic aldehydes showed a loose roaming saddle point along the roaming dissociation coordinate[26,27], whereas the roaming signature in methyl formate (HCOOCH3) was verified to involve multiple energy states via a conical intersection[17,18]. These two types of roaming dynamics apparently behave differently.
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