Markus Gühr

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 guests 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.

Controlling vibrational wave packet revivals in condensed phase: Dispersion and coherence for Br2 in solid Ar

Interferences in vibrational wave packets of Br2 molecules are controlled in the presence of a solid Ar environment that provides decoherence. By applying a negatively or positively chirped excitation pulse, one can set the clock backward, respectively forward, in the wave packet propagation. Based on this mechanism, we present a general scheme to record vibrational decoherence. Wave packets are spatially focused at Topt by applying negatively chirped pulses. From the focussing contrast, we determine a vibrational dephasing time on the B state of Tvibdeph n = 3 ps. We use positively chirped pulses to bring the formation of fractional revival structures forward with respect to Tvibdeph. By exciting four vibrational levels with such a pulse, we observe a 1/6 revival indicating the vibrational coherence time T4deph for exactly four levels. The required chirp prolongs the pulse duration by a factor of ten to Δτ n = 300 fs. Electronic dephasing Teldeph restricts the revival control efficiency to parts of the pulse with Δτ 300 fs.

Depolarization as a probe for ultrafast reorientation of diatomics in condensed phase: ClF versus I2 in rare gas solids

Polarization dependent femtosecond pump-probe spectra display characteristic vibrational wave packet dynamics of ClF in Ar (isotropic cage and small fragment size) and I2 in Kr (cylindrical cage and large fragments). The intensity ratio of the signals for pumping with parallel versus crossed polarization with respect to the probe pulse is close to the value 1/3, as expected for full photoselection immediately after excitation. For ClF this ratio depolarizes to unity within τr=1.2u2009ps, showing the ultrafast randomization of the orientation of the molecular bond due to fragment scattering off the matrix cage. The direction of the I2 bond is geometrically fixed by the Kr matrix and the ratio remains constant.

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.

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.

A potential energy surface and a trajectory study of photodynamics and strong-field alignment of ClF molecule in rare gas (Ar,Kr) solids

Molecular alignment by a strong nonresonant laser pulse and a subsequent ultrafast pump–probe experiment are investigated by classical molecular dynamics simulations. A tendency to molecular alignment in the solid host in the presence of an intense ultrashort laser field is examined by simulating the external force due to the polarizability - field interaction. The ground state rotational potential of the ClF molecule is evaluated as the key determining factor along with the dynamics. The pump–probe scheme consists of time-delayed excitations X 1Σ+0u2006→u2006(B 3Π0 or 1Π1)u2006→u2006ionic states of ClF molecule in Ar or Kr crystals. We show the yields of dissociating ClF trajectories and discuss the fate of cage-exiting F atoms as a competing event to geminate recombination. Emphasis is put on the recombining trajectories. We extract the round-trip times and reorientations for the excited state “wave-packet” motion as important parameters for the analysis of the experimental photodynamics, and show the surrounding lattice response to the collisions.

Femtosecond spectroscopy of fragment–cage dynamics: I2 in Kr

We report femtosecond pump–probe spectra for the B state of I2 in solid Kr with systematic variation of both pump and probe wavelength. The observed oscillations strongly depend on the probe wavelength and we show that it must be tuned together with the pump pulses to obtain the correct vibrational frequencies. We construct an RKR potential for the B state that includes the solvent response. The ionic E state surface is directly constructed from the measurements. Both are compared to DIM potentials. We report rates for vibrational energy relaxation in the B state that increase by three orders of magnitude on going from low excitation to excitation beyond the gas phase dissociation limit. By systematic variation of the probe window we record snapshots of a mean trajectory at the dissociation limit that clearly displays the strong interaction with the cage.

Photodynamics and ground state librational states of ClF molecule in solid Ar. Comparison of experiment and theory

Photodynamics calculations of a ClF molecule in solid Ar are compared to experimental results and a new interpretation is given for the observed femtosecond-pump–probe signal modulation. We analyze the round-trip and depolarization times for the excited state wave-packet motion and discuss the incorporation of lattice cage motions that partially explain the time dependence of the measured signal. Librational eigenstates and eigenenergies are calculated by solving the rotational Schrodinger equation in the previously computed [T. Kiljunen, M. Bargheer, M. Guhr and N. Schwentner, Phys. Chem. Chem. Phys., 2004, 6, 2185–2197] octahedral potentials that hinder free molecular rotation in the solids. The obtained level structure is compared to infrared-spectroscopic results. We comment on the correspondence between temperature effects in the classical dynamics of the nuclei and the quantum mechanical probability distributions. We find the combinative treatment of different simulation temperatures congruous for interpreting the experimental results at cryogenic conditions.

Collisions transfer coherence

Ultrafast photoexcitation of molecules prepares coherent superpositions of vibrational states, forming wave packets. We show direct experimental evidence that impulsive collisions of photo-fragments with large energy lossgenerate new coherent superpositions of vibrational states, causing coherence among previously unoccupied states. We demonstrate the effect in the model system of I 2 molecules in Kr solids. This coherence transfer-even in the presence of very strong dissipation-may be used for control strategies in condensed-phase photoreactions.

Wavepacket interferometry and wavepacket dynamics in condensed phase

The diatomic Cl2 in solid Ar is examined to discuss the information obtained by wavepacket interferometry in Condensed phase and to contrast it with femtosecond pump-probe and linear absorption spectra.