Cage motions induced by electronic and vibrational excitations: Cl2 in Ar

Quantum dynamics and spectroscopy of dihalogens in solid matrices. I. Efficient simulation of the photodynamics of the embedded I2Kr18 cluster using the G-MCTDH method

The molecular dynamics following the electronic BΠu30⁺⟵XΣg+1 photoexcitation of the iodine molecule embedded in solid krypton are studied quantum mechanically using the Gaussian variant of the multiconfigurational time-dependent Hartree method (G-MCTDH). The accuracy of the Gaussian wave packet approximation is validated against numerically exact MCTDH simulations for a fully anharmonic seven-dimensional model of the I₂Kr₁₈ cluster in a crystal Kr cage. The linear absorption spectrum, time-evolving vibrational probability densities, and I₂ energy expectation value are accurately reproduced by the numerically efficient G-MCTDH approach. The reduced density matrix of the chromophore is analyzed in the coordinate, Wigner and energy representations, so as to obtain a multifaceted dynamical view of the guest-host interactions. Vibrational coherences extending over the bond distance range 2.7 Å < R_I-I < 4.0 Å are found to survive for several vibrational periods, despite extensive dissipation. The present results prepare the ground for the simulation of time-resolved coherent Raman spectroscopy of the I₂-krypton system addressed in Paper II.

Quantum dynamics of solid Ne upon photo-excitation of a NO impurity: a Gaussian wave packet approach

A high-dimensional quantum wave packet approach based on Gaussian wave packets in Cartesian coordinates is presented. In this method, the high-dimensional wave packet is expressed as a product of time-dependent complex Gaussian functions, which describe the motion of individual atoms. It is applied to the ultrafast geometrical rearrangement dynamics of NO doped cryogenic Ne matrices after femtosecond laser pulse excitation. The static deformation of the solid due to the impurity as well as the dynamical response after femtosecond excitation are analyzed and compared to reduced dimensionality studies. The advantages and limitations of this method are analyzed in the perspective of future applications to other quantum solids.

Vibronic dynamics of I(2) trapped in amorphous ice: coherent following of cage relaxation

Four-wave mixing measurements are carried out on I(2)-doped ice, prepared by quench condensing the premixed vapor at 128 K. Coherent vibrational dynamics is observed in two distinct ensembles. The first is ascribed to trapping in asymmetric polar cages in which, as in water, the valence absorption of the molecule is blueshifted by 3500 cm(-1), predissociation of the B state is complete upon the first extension of the molecular bond, and the vibrational frequency in the ground state (observed through coherent anti-Stokes Raman scattering) is reduced by 6.5%. The effect is ascribed to polarization of the molecule. The implied local field and the ionicity of the molecule are extracted, to conclude that the molecule is oxygen bonded to one water molecule on one side and hydrogen bonded on the other side. The second ensemble is characterized by the transient grating signal, which shows coherent vibrational dynamics on the B state. The small predissociation rate in this site suggests a symmetric cage in which the local electric field undergoes effective cancellation; and consistent with this, the extracted blueshift of the valence transition in this site (approximately 1500 cm(-1)) coincides with that observed in clathrate hydrates of iodine. Remarkably, in this site, the vibrational period of the B state packet coherently stretches from an initial value of 245 fs to 325 fs in the course of five oscillations (1.3 ps), indicative of vibrationally adiabatic following of the cage expansion. The dynamics is characteristic of a molecule trapped in a tight symmetric cage, with a soft cage coordinate that relaxes without eliciting elastic response. Enclathration in low-density amorphous ice is concluded.

Ultrafast dynamics of halogens in rare gas solids

We perform time resolved pump-probe spectroscopy on small halogen molecules ClF, Cl2, Br2, and I2 embedded in rare gas solids (RGS). We find that dissociation, angular depolarization, and the decoherence of the molecule is strongly influenced by the cage structure. The well ordered crystalline environment facilitates the modelling of the experimental angular distribution of the molecular axis after the collision with the rare gas cage. The observation of many subsequent vibrational wave packet oscillations allows the construction of anharmonic potentials and indicate a long vibrational coherence time. We control the vibrational wave packet revivals, thereby gaining information about the vibrational decoherence. The coherence times are remarkable larger when compared to the liquid or high pressure gas phase. This fact is attributed to the highly symmetric molecular environment of the RGS. The decoherence and energy relaxation data agree well with a perturbative model for moderate vibrational excitation and follow a classical model in the strong excitation limit. Furthermore, a wave packet interferometry scheme is applied to deduce electronic coherence times. The positions of those cage atoms, excited by the molecular electronic transitions are modulated by long living coherent phonons of the RGS, which we can probe via the molecular charge transfer states.

Persistent nuclear wave packet oscillation coexistent with incoherent vibrational population at excited F centers in KI

We investigated nuclear wave packet dynamics in the excited state of KI F centers at 10 K using time-resolved luminescence spectroscopy. Observed transient spectrum is divided into oscillatory and non-oscillatory components. The former lasts over 11 ps without appreciable damping and is attributed to the oscillation of the wave packet consisting mainly of the A(1g) mode around the center. The non-oscillatory part rises quickly after photo-excitation exhibiting a cooling of incoherent vibrational population. This behavior suggests the fast energy dissipation due to the dephasing of the bulk phonon modes.