Non-Coulomb Explosions of Molecules in Intense Laser Fields

Femtosecond Laser Filamentation

“…He did not consider the physical and chemical processes leading to fluorescence except to assume that after fragmentation, there would be fluorescence. It turns out that the fluorescence is at least partially related to the excitation of the superexcited states of the molecules (Kong et al, 2006;Kong and Chin, 2008). Superexcited states are defined as those molecular states whose total internal energy is higher than the first ionization energy of the molecule.…”

“…On the other hand, intense femtosecond laser pulses, when interacting with molecules, would lead to multiphoton/tunnel ionization, fragmentation, Coulomb explosion, etc. Excitation of the superexcited states was never considered until recently (Kong et al, 2006;Kong and Chin, 2008). In fact, they realized that the fluorescence spectra from CH 4 excited by synchrotron radiation and by ultrafast intense Ti-sapphire laser pulses are almost identical; i.e., they both yield the fluorescence bands of CH radicals corresponding to the transitions of A 2 , B 2 − , C 2 + → X and the Balmer α-line of H (n = 3).…”

Population inversion in fluorescing fragments of super-excited molecules inside an air filament

“…[15] All three kinds of hydrocarbons could be fragmented via the excitation through the SES of the molecules inside the filament, resulting in CH fluorescence in each case. [3,16] The fluorescence spectra for the mixture of air with ethylene (C 2 H 4 ) and air with methane (CH 4 ) have also been measured. They both show the fluorescence of CH at 431 nm (not shown).…”

“…[4] The fluorescence from the fragments CH of these molecules has been recognized to come from super-excitation by the 800-nm femtosecond laser pulse. [3,16] A qualitative description of this population inversion during strong field superexcitation of molecules is given below. [17,18] C 2 H 2 in air.…”

Femtosecond laser ionization and fragmentation of molecules for environmental sensing

“…Despite the high potential of femtosecond laser ionization and fragmentation of molecules in view of applications in environmental sensing, the underlying mechanisms behind the fingerprint emissions of molecules in ambient air are quite complicated, which leave a lot of challenges ahead. On the one hand, when molecules are exposed to such high intensity of 10 13 –10 14 W/cm 2 , ionization and fragmentation (dissociation) of molecules occur rapidly with the time scale comparable to the laser pulse duration, resulting in fluorescing ions or fragments directly . Ideally speaking, ultrafast ionization and fragmentation dynamic of molecules in intense laser fields could be best investigated in a collision‐less, hence, vacuum condition by probing electron, ion and photon signals simultaneously.…”