Cross sections for rotational decoherence of perturbed nitrogen measured via decay of laser-induced alignment.

We quantitatively determine cross sections for rotational decoherence from the decay of nonadiabatic laser-induced alignment in nitrogen and nitrogen-foreign gas mixtures in a temperature range between 80 K and room temperature. The cross section for rotational decoherence in pure nitrogen decreases from 102 A(2) at 80 K to 48 A(2) at 295 K, leading to long-lived coherences even at high temperatures. Comparison with the broadening of the transition lines of the Raman Q-branch reported in the literature shows that the decay of rotational coherence proceeds at the same rate as rotational depopulation. This is verified also for mixtures of nitrogen with hydrogen, helium, argon, and krypton. We discuss limits posed by a possible J-dependence of the cross sections and strategies for state resolved determination from the time-dependent alignment signal.

Dynamics of electronic states and spin-flip for photodissociation of dihalogens in matrices: experiment and semiclassical surface-hopping and quantum model simulations for F2 and ClF in solid Ar.

Three approaches are combined to study the electronic states' dynamics in the photodissociation of F(2) and ClF in solid argon. These include (a) semiclassical surface-hopping simulations of the nonadiabatic processes involved. These simulations are carried out for the F(2) molecule in a slab of 255 argon atoms with periodic boundary conditions at the ends. The full manifold of 36 electronic states relevant to the process is included. (b) The second approach involves quantum mechanical reduced-dimensionality models for the initial processes induced by a pump laser pulse, which involve wavepacket propagation for the preoriented ClF in the frozen argon lattice and incorporate the important electronic states. The focus is on the study of quantum coherence effects. (c) The final approach is femtosecond laser pump-probe experiments for ClF in Ar. The combined results for the different systems shed light on general properties of the nonadiabatic processes involved, including the singlet to triplet and intertriplet transition dynamics. The main findings are (1) that the system remains in the initially excited-state only for a very brief, subpicosecond, time period. Thereafter, most of the population is transferred by nonadiabatic transitions to other states, with different time constants depending on the systems. (2) Another finding is that the dynamics is selective with regard to the electronic quantum numbers, including the Lambda and Omega quantum numbers, and the spin of the states. (3) The semiclassical simulations show that prior to the first "collision" of the photodissociated F atom with an Ar atom, the argon atoms can be held frozen, without affecting the process. This justifies the rigid-lattice reduced-dimensionality quantum model for a brief initial time interval. (4) Finally, degeneracies between triplets and singlets are fairly localized, but intertriplet degeneracies and near degeneracies can span an extensive range. The importance of quantum effects in photochemistry of matrix-isolated molecules is discussed in light of the results.

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.

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

Femtosecond dynamics of molecular vibrations as well as cage motions in the B<--X transition of Cl2 in solid Ar have been investigated. We observed molecular vibrational wave-packet motion in experimental pump-probe spectra and an additional oscillation with a 500 fs period which is assigned to the zone-boundary phonon of the Ar crystal. The cage motion is impulsively driven by the B<--X transition due to the expansion of the electronic cloud of the chromophore. To clarify the underlying mechanism, we performed simulations based on the diatomics-in-molecules method which takes into account the different shapes of the Cl2 electronic wave function in the B and X states as well as the anisotropic interaction with the matrix. The simulation results show that Ar atom motion in the (100) plane is initiated by the electronic transition and that only those Ar atoms oscillate coherently with an approximately 500 fs period which are essentially decoupled from the molecular vibration. Their phase and time evolution are in good agreement with the experimentally observed oscillation, supporting the assignment as a displacive excitation of coherent phonons.

Pump-probe spectroscopy with phase-locked pulses in the condensed phase: decoherence and control of vibrational wavepackets.

Electronic and vibrational coherences of Cl2 embedded in solid Ar are investigated by exciting to the B state with a phase-locked pulse pair from an unbalanced Michelson interferometer, where the chirp difference matches the B state anharmonicity. Recording the A' --> X fluorescence after relaxation is compared to probing to charge transfer states by a third pulse. The three-pulse experiment delivers more details on the decoherence processes. The signal modulation due to phase tuning up to the third vibrational round-trip time indicates that the electronic coherence in the B <-- X transition is preserved for more than 660 fs in the solid Ar environment where many body electronic interactions take place. Vibrational coherence lasts longer than 3 ps according to the observed half revival of the wavepacket. Control of the coupling between wavepacket motion and lattice oscillation is demonstrated by tuning the relative phase between the phase-locked pulses, preparing wavepackets predominantly composed of either zero-phonon lines or phonon side bands.

Coherent phonon dynamics: Br2 in solid Ar.

A long lasting coherent oscillation with a sharp frequency of f(p) = 2 THz is observed in fs pump probe spectra for B <-- X excitation of Br2 in solid argon. It exactly matches the frequency of a coherent zone boundary phonon (ZBP) of the Ar environment. The ZBPs have a vanishing group velocity v(g), thus they stay in the vicinity of the chromophore. They originate from a displacive excitation of coherent phonons (DECP) initiated in the electronic B <-- X transition, because neither f(p) nor the phase of the oscillation do depend on the B state vibrational dynamics. A model calculation shows that an expansion of the electronic density in going from the electronic ground state X to the B state kicks the Ar atoms in the Br2 vicinity. In addition, a group of Ar atoms in the (100) plane is decoupled from the intramolecular dynamics afterwards. The ZBP modulates the solvation energy of the terminal charge transfer states used in the probe transition from the B state and thus the detection sensitivity. The contrast is enhanced by probing the B state wave packet with the cutting edge of the probe window. This is in full accordance with a study for I2 : Kr.

Effective chromophore potential, dissipative trajectories, and vibrational energy relaxation: Br2 in Ar matrix.

The intramolecular wave packet dynamics on the electronic B (3pi0) potential of Br2 in solid argon is induced and interrogated by femtosecond pump-probe spectroscopy. An effective potential of the chromophore in the solid is derived from the wave packet period for different excitation photon energies. Deep in the potential well, it is consistent with vibrational energies from wavelength-resolved spectra. It extends to higher energies, where the vibrational bands merge to a continuum, and even beyond the dissociation limit, thus quantifying the cage effect of the argon matrix. This advantage of pump-probe spectroscopy is related to a reduced contribution of homogeneous and inhomogeneous line broadenings. The vibrational energy relaxation rates are determined by a variation of the probe window spatial position via the probe quantum energy. A very large energy loss in the first excursion of the wave packet is observed near the dissociation limit. This strong interaction with the argon matrix is directly displayed in an experimental trajectory.

Photochemistry in the charge transfer and neutral excited states of HCl in Xe and Kr matrices.

HCl-doped Xe and Kr films are irradiated with wavelength dispersed synchrotron radiation in the wavelength range from 200 to 130 nm. The growth of H, Cl, Xe2H+, XeH2, HXeCl, Kr2H+, and HKrCl as well as the decomposition of HCl are recorded by a combination of UV, VIS, and IR spectroscopy. A turnover in the formation of Xe2H+ and Kr2H+ by a predominant two-step reaction on neutral surfaces at low energies to a one-step formation on ionic surfaces is determined at 172 and 155 nm in Xe and Kr, respectively. A potential energy diagram for neutral and ionic states is derived that is consistent with a DIIS calculation, with new UV fluorescence bands from Xe+HCl- centers, with the turnover energies and with a deconvolution of the absorption spectra in neutral and ionic contributions. The cage exit of charged as well as of neutral H, the latter via a harpoon reaction, is discussed for the ionic surfaces. The self-limitation of HCl decomposition on the neutral surfaces due to absorption by H and Cl fragments is treated quantatively. Dissociation efficiencies phi(e), together with absolute absorption cross sections sigma(H) and sigma(Cl) of the fragments, are derived. sigma(H) and sigma(Cl) are of the order of 10(-16) cm(2) compared to 10(-18) cm(2) for sigma(HCl). Dissociation is accompanied by many excitation cycles of the fragments, which leads to light-induced migration of H and recombination. phi(e) therefore represents a product of the cage exit probability phi that was treated theoretically and the survival probability concerning geminate and nongeminate recombination.

Generation of coherent zone boundary phonons by impulsive excitation of molecules.

A coherent zone boundary phonon (ZBP) of solid Kr (f(m)=1.54 THz) is observed in ultrafast pump-probe spectra of I2 guest molecules in a Kr crystal. Its phase is stable for at least 10 ps. The femtosecond pump pulse induces an electronic transition in I2. The resulting expansion of the guest's electronic cloud impulsively excites phonons in the host. Detection at the impurity after some picoseconds selects the ZBP due to its low group velocity. A ZBP amplitude of 0.02 A is estimated.

Ultrafast solvent-induced spin-flip and nonadiabatic coupling: ClF in argon solids.

Femtosecond pump-probe spectra show direct evidence for ultrafast solvent-induced spin flip in photodissociation-recombination events of ClF, a light diatomic molecule, for which the spin-orbit coupling is weak. The bound triplet states ((3)Pi) of ClF are probed and the dynamics for excitation to the singlet state ((1)Pi(1)) is compared with excitation to the triplet state B((3)Pi(0)). The population initially excited to the singlet state (1)Pi(1) is transferred to the bound triplet states (3)Pi within tau(f)=0.5 ps. Oscillations in the spectra indicate wave packet dynamics with the triplet state period of 300 to 400 fs in both cases. According to simulations of F(2)/Ar, most of the initially excited singlet state population is converted to repulsive and weakly bound triplet states within approximately 60 fs. In the first ps, 40% of the triplet population accumulates in the weakly bound (3)Pi states, in good accord with the experiment.

Dynamics of localized excitations from energy and time resolved spectroscopy.

The electronic structure and the excited state relaxation phenomena in condensed rare gases have been studied by absorption spectroscopy, photoelectron spectroscopy, and time resolved luminescence spectroscopy, all using selective excitation by monochromatized synchrotron radiation. Radiationless and radiative electronic transitions in excited states and the rearrangement of surrounding matrix atoms will be discussed for excited rare gas atoms in rare gas matrices. Furthermore, experimental results concerning the scattering of excitons within the free exciton branches, localization of excitons, relaxation within the self-trapped exciton states, and energy transfer to guest atoms and boundaries will be presented.