Femtosecond Time-Resolved Pump−Probe Spectroscopy of NaI in Rare-Gas Environment

Abstract

In this paper we report femtosecond studies of the influence of rare-gas collisions on the dissociation reaction of NaI molecules by investigating the coherent wave packet motion in NaI at a temperature of 670 °C and rare-gas pressures ranging from 0 to 1000 bar. The photodissociation dynamics of NaI is influenced by the interaction between the excited covalent and the ionic ground state potential energy surfaces, which cross at a certain internuclear separation. Due to the adiabaticity of the so formed potential, studies of the elementary nuclear motion and reaction dynamics along the covalent (Na + I) and the ionic (Na+ + I-) channels are possible. The probing is made for the transition-state complex [Na···I]⧧*. Even at low rare-gas pressures a considerable influence of collisions on the wave packet motion within the adiabatic potential well is found. Only few collisions between NaI molecules and surrounding rare-gas atoms result in fast vibrational relaxation and loss of coherence. Furthermore, a stabilization of the transition state is observed due to the decrease of the Landau−Zener escape probability. The reaction cross section for the interaction of the transition state molecules with helium atoms is found to be smaller than for the interaction with argon atoms. The intensity of the detected laser-induced fluorescence arising from the transition Na ← Na* decreases with increasing rare-gas pressure. Here, the effect for argon atoms is smaller than for helium atoms.

Geminate recombination processes induced by rare gas collisions with predissociating NaI molecules prepared by femtosecond excitation

“…The spatial distribution of the rare-gas atoms is assumed to be isotropic and the velocities are assumed to obey a Maxwell-Boltzmann distribution for a temperature of 960 K [1]. The spatial distribution of the rare-gas atoms is assumed to be isotropic and the velocities are assumed to obey a Maxwell-Boltzmann distribution for a temperature of 960 K [1].…”

“…The density of NaI is assumed to be negligible [1]. At each timestep Dt we choose at random the velocities of the rare gas atoms according to the Maxwell-Boltzmann statistics and calculate the collision probability .…”

“…In recent experiments the predissociation of NaI in rare gas environments was investigated under high-pressure conditions with the help of time-resolved femtoscond spectroscopy [1]. It was found that the oscillating transient signals which re¯ect the gas-phase predissociation dynamics [2±4] are destroyed with increasing pressure.…”

Femtosecond time-resolved spectroscopy of elementary molecular dynamics

“…Many other fields have since been added, such as investigations of the relaxation dynamics in liquid phase (Kaiser 1988;Zinth et al 1988) and ultrafast spectroscopy of semiconductors (Shah 1996). In order to understand the elementary processes which take place in molecules after coherent excitation into particular electronic and/or ro-vibrational states it is advantageous first to perform investigations on isolated molecules Materny et al 1996;Liu et al 1996;Knopp et al 1997). On a sub-nanosecond time scale, vapor phase or molecular beams offer collision-free conditions.…”

Real-time control of electronic motion: Application to NaI

“…Thus the oscillating electron distribution is ''caught'' and ''frozen'' by the laser pulse that breaks the bond. The dynamics of the NaI molecule has been studied extensively, experimentally [9][10][11][12][13] as well as theoretically. [14][15][16][17][18][19][20] The potential energy surfaces, the nonadiabatic coupling and the dipole moments are known.…”