Probing Ultrafast Dissociation Dynamics of Chloroiodomethane in the B Band by Time-Resolved Mass Spectrometry

Ultrafast dissociation dynamics of chloroiodomethane (CH_2ICl) in the B band is studied by femtosecond time-resolved time-of-flight (TOF) mass spectrometry. Time-resolved TOF mass signal of parent ion (CH_2ICl^+) and main daughter ion (CH_2Cl^+) are obtained. The curve for the transient signal of CH_2ICl^+ is simple and can be well fitted by an exponential decay convoluted with a Gaussian function. The decay constant determined to be less than 35 fs reflects the lifetime of the B band. Significant substituent effects on photodissociation dynamics of CH_2ICl compared with CH_3I are discussed. The dissociation time from the parent ion CH_2ICl^+ to the daughter ion CH_2Cl^+ is determined in the experiment. The optimized geometry of the ionic state of CH_2ICl and the ionization energy are calculated for further analysis of the measurements. In addition, compared with the parent ion, a new decay component with time constant of \sim 596 fs is observed for CH_2Cl^+, and reasonable mechanisms are proposed for the explanation.