Excited-state dynamics of m-dichlorobezene in ultrashort laser pulses

Abstract

The excited state dynamics of aromatic hydrocarbon has attracted a great deal of attention due to its important role in photophysics and atmosphere chemistry. With the benefit of ultra-short laser pulses, the ultrafast phenomenon can be studied in a time resolved way. In the present work, m-dichlorobenzene, a typical model of aromatic hydrocarbon, is investigated by the femtosecond time resolved time-of-flight mass spectroscopy. In order to reveal its excited state dynamics, m-dichlorobenzene is pumped to the excited state after absorbing one 200/267 nm photon, and then ionized by absorbing 800 nm photons. Time resolved mass spectra are recorded with time of flight. At 200 nm, m-dichlorobenzene is excited to a (, *) state. Three decay components are observed in the transient profiles of m-dichlorobenzene ions, which correspond to three competition channels in the excited states. The first channel is an ultrafast dissociation process via a repulsive state with (n, *) or (, *) character, and the lifetime is (0.150.01) ps. The second channel is an internal conversion process from the populated excited state to the hot ground state, and the lifetime of the redistribution of the internal vibration in the hot ground state is (4.940.08) ps. The third channel is an intersystem crossing process to the triplet state, and the lifetime is (110.094.33) ps. Moreover, the transient profiles of C6H4Cl+/C6H4+ display similar decay tendencies to the transient profile of parent ion, except that longer lifetime constants ((127.3829.29) ps for C6H4Cl+, and (123.7637.12) ps for C6H4+, respectively) are observed. It is likely that the fragment ions result from the dissociative ionization of the parent molecule. At 267 nm, m-dichlorobenzene is excited to the first excited state with (n, *) character. Only C6H4Cl2+ and C6H4Cl+ are observed in the two-color mass spectrum. A slow decay component (~(1.060.05) ns) is obtained for both the parent ion and the fragment ion. It is attributed to an intersystem crossing process from the first excited state S1 to the triplet state T1. Furthermore, the transient profile of C6H4Cl+ displays other decay components, i.e., (2.480.09) ps, in addition to the slow decay component. This fast decay process can be attributed to an internal conversion process from the populated excited states to the hot ground states. The present study provides a more in-depth understanding of the ultrafast excited state dynamics of m-dichlorobenzene.