The I*(2P1/2)−I*(2P1/2) Contact Pair Emission in Condensed Media: A Molecular Spring-Gauge for Cavity Sizing

Electronic spectroscopy of I(2)-Xe complexes in solid Krypton

In the present work, we have studied ion-pair states of matrix-isolated I(2) with vacuum-UV absorption and UV-vis-NIR emission, where the matrix environment is systematically changed by mixing Kr with Xe, from pure Kr to a more polarizable Xe host. Particular emphasis is put on low doping levels of Xe that yield a binary complex I(2)-Xe, as verified by coherent anti-Stokes Raman scattering (CARS) measurements. Associated with interaction of I(2) with Xe we can observe strong new absorption in vacuum-UV, redshifted 2400 cm(-1) from the X → D transition of I(2). Observed redshift can be explained by symmetry breaking of ion-pair states within the I(2)-Xe complex. Systematic Xe doping of Kr matrices shows that at low doping levels, positions of I(2) ion-pair emissions are not significantly affected by complexation with Xe, but simultaneous increase of emissions from doubly spin-excited states indicates non-radiative relaxation to valence states. At intermediate doping levels ion-pair emissions shift systematically to red due to change in the average polarizability of the environment. We have conducted spectrally resolved ultrafast pump-probe ion-pair emission studies with pure and Xe doped Kr matrices, in order to reveal the influence of Xe to I(2) dynamics in solid Kr. Strikingly, relaxed emission from the ion-pair states shows no indication of complex presence. It further indicates that the complex escapes detection due to a non-radiative relaxation.

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.

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.

Dynamics and the breaking of a driven cage: I2 in solid Ar

Pump-probe measurements of I2 in solid Ar are reported and analyzed to extract a description of cage response to impulsive excitation, from the gentle kick, up to the breaking point. The most informative data are obtained through wavepacket motion on cage-bound, but otherwise dissociative, potentials where the chromophore acts as a transducer to drive the cage and to report on the local dynamics. This general class of dynamics is identified and analyzed as a function of energy in Ar, Kr, and Xe. The overdriven cage rebounds with a characteristic period of 1.2 ps that shows little dependence on excitation amplitude in all hosts. After rebound, the cage rings as a local resonant mode in Ar, with a period of 1 ps and dephasing time of 3 ps. This mode remains at the Debye edge in Kr and Xe, with periods of 630 and 800 fs, and dephasing times of 8 and 6 ps, respectively. In the bound B-state, the cage fluctuates toward its dilated equilibrium structure on a time scale of 3 ps, which is extracted from the down-chirp in the molecular vibrational frequency. When kicked with excess energy of 4 eV, the Ar cage breaks with 50% probability, and the molecule dissociates. The kinetics of polarization selective, multiphoton dissociation with Gaussian laser intensity profiles is delineated and the ballistics of cage breakout is described: The photodissociation proceeds by destruction of the local lattice, by creating interstitials and vacancies. During large amplitude motion on cage-bound potentials, sudden, nonadiabatic spin-flip transitions can be observed and quantified in space and time. The spin-flip occurs with unit probability in Ar when the I*-I bond is stretched beyond 6 A.