Photodissociation/photoionization processes of chlorobromomethane induced by femtosecond laser pulses with pump-probe scheme

Abstract

The photodissociation/photoionization processes of chlorobromomethane (CH 2 BrCl) induced by femtosecond laser pulses have been investigated using pump-probe scheme combined with the time-of-flight mass spectra. The dominate photoproducts are observed at different delay time of the pump (400 nm) and probe (800 nm) pulses and the corresponding time-dependence of them is obtained. The results show that the decaying time of the molecule CH 2 BrCl in the A-band is in the 100 fs. The decaying tendencies of the fragment ions (CH 2 Cl + and CH 2 Br + ) and the parent ion (CH 2 BrCl + ) are almost the same and the relative ratios of the yields of them keep constant during the delay time of 0 to 150 fs. These facts suggest that the fragment ions come from the fragmentation of the parent ions in excited electronic states. The probabilities to form CH 2 Cl + and CH 2 Br + are obtained from the relative ratio of the ion intensity and are about 71.6% and 14.2%, respectively. chlorobromomethane, femtosecond, pump-probe, photodissociation, photoionizationUnderstanding of the photochemistry of halomethanes has received significant attention in recent decades due to two main reasons. On one hand, the halogen atoms and halogen-containing radicals have been realized as chemical species for the destruction of stratospheric ozone [1][2][3] ; on the other hand, the halomethanes have been chosen as prototypes for mode specific photochemistry upon laser excitation [4,5] .The chlorobromomethane, as a typical monohalomethane, has been extensively studied from experimental [5][6][7][8] and theoretical [9][10][11] point of view as an ideal research object in the photochemistry field. Now, the general features of its photodissociation dynamics in ultraviolet region have been well understood. Compared with CH 3 I molecule [4] , photodissociation of CH 2 BrCl molecule in ultraviolet region involves more states because of the lowering of the molecular symmetry from C 3v to C s , which results in the splitting of the degenerate states, 3 Q 1 and 1 Q 1 , into [2A′, 1A″] and [4A′, 2A″], respectively. The 3 Q 0 state in C 3v symmetry becomes the 3A' states in C s symmetry. The 3A′ state diabatically correlates to the CH 2 Cl+Br*( 2 P 1/2 ) products, but an avoid crossing occurs between the 3A′ and 4A′ states, resulting in an adiabatic correlation of the 3A′ to the CH 2 Cl+Br( 2 P 3/2 ) products and the 4A′ state to the CH 2 Cl+Br*( 2 P 1/2 ) products. The other states 2A′, 1A″ and 2A″ correlate adiabatically the ground state products CH 2 Cl+Br( 2 P 3/2 ). In the 193-267nm region, however, no breaking of C-Cl bonds happen when CH 2 BrCl molecules absorb a single ultraviolet photon [8] .With the advent of the advanced femtosecond laser technology [12] , now the combination of ultrashort laser

Systematic Trends in Photonic Reagent Induced Reactions in a Homologous Chemical Family

“…The latter feature has been previously reported in TOF spectra of CH 2 BrCl, CH 2 ICl, CH 2 IBr, and CH 2 I 2 obtained from interaction with 800 nm TL pulses. 20,21,31,32,46 The only exception is CH 2 FCl, where the M + yield is greater than that of the fragment CH 2 F + . This property arises because the appearance energy for CH 2 F + lies much higher above the IE of CH 2 FCl as compared to other molecules (cf.…”

“…The central question considered in this work is whether systematic variation in observable yields analogous to Figure b occurs when the chemical reagents are replaced by photonic reagents. Ultrafast laser-induced dissociative ionization reactions using a set of photonic reagents with a fixed substrate or vice versa have been explored for many classes of molecules. − These studies revealed detailed information about the physical mechanisms underlying the dissociation dynamics in individual molecules and/or identified specific photonic reagents that produce a desired yield of some observable, typically a particular photoproduct or ratio of photoproducts. In contrast, less attention has been given to examining families of chemically homologous molecules interacting with families of photonic reagents; previous investigations typically have examined only a few related molecules − or a large set of unrelated molecules .…”