Sanford Ruhman

IMPULSIVE EXCITATION OF COHERENT VIBRATIONAL MOTION GROUND SURFACE DYNAMICS INDUCED BY INTENSE SHORT PULSES

A framework for understanding impulsively photoinduced vibrational coherent motion on the ground electronic surface is presented. In particular strong resonant excitation to a directly dissociative electronic surface is considered. Three distinct approaches are employed. A two surface Fourier wavepacket method explicitly including the field explores this process in isolated molecules. A coordinate dependent two‐level system is employed to develop a novel analytical approximation to the photoinduced quantum dynamics. The negligible computational requirements make it a powerful interactive tool for reconstructing the impulsive photoexcitation stage. Its analytical nature provides closed form expressions for the photoinduced changes in the material. Finally the full simulation of the process including the solvent effects is carried out by a numerical propagation of the density operator. In all three techniques the excitation field is treated to all orders, allowing an analysis of current experiments using st...

Spectroscopic tracking of structural evolution in ultrafast stilbene photoisomerization.

Understanding a chemical reaction ultimately requires the knowledge of how each atom in the reactants moves during product formation. Such knowledge is seldom complete and is often limited to an oversimplified reaction coordinate that neglects global motions across the molecular framework. To overcome this limit, we recorded transient impulsive Raman spectra during ultrafast photoisomerization of cis-stilbene in solution. The results demonstrate a gradual frequency shift of a low-frequency spectator vibration, reflecting changes in the restoring force along this coordinate throughout the isomerization. A high-level quantum-chemical calculation reproduces this feature and associates it with a continuous structural change leading to the twisted configuration. This combined spectroscopic and computational approach should be amenable to detailed reaction visualization in other photoisomerizing systems as well.

Ultrafast photodissociation of I3. Coherent photochemistry in solution

We report a comprehensive study of the UV photolysis of I3− in ethanol solution, using femtosecond time resolved transient transmission experiments. We interpret our results to indicate that with high probability, photoexcitation leads to direct formation of di‐iodide ions within 300 fs, which are vibrating coherently. Through our experiments we have been able to determine that the time scales for vibrational dephasing, vibrational relaxation, and reorientation of the fragment ions are 400 fs, 4 ps, and 5 ps, respectively. Transmission signals at 620 nm and at 880 nm, which are above and below the λmax of the known absorption of I2−, oscillate at a precisely opposite phase. This and other results presented indicate that through the oscillations we are observing coherent vibration of the I2− photofragment. UV transient transmission experiments have been conducted in order to characterize the time scales for recombination. Preliminary results show that recombination takes place on several time scales. A fas...

On the Absence of Detectable Carrier Multiplication in a Transient Absorption Study of InAs/CdSe/ZnSe Core/Shell1/Shell2 Quantum Dots

Tunable femtosecond pump-near IR probe measurements on InAs/CdSe/ZnSe core/shell1/shell2 nanocrystal quantum dots were conducted to quantify spontaneous carrier multiplication previously reported in this system. Experimental conditions were chosen to eliminate the need for determining absolute wavelength dependent cross sections of the nanocrystals and allow direct comparison of band edge absorption bleach kinetics for different excitation energies up to 3.7 times the band gap. Results for two sample sizes show no signs of carrier multiplication within that range. This result is discussed in light of reports describing occurrence of this novel phenomenon in quantum dots based on this as well as numerous other semiconductor materials.

Coherent molecular vibrational motion observed in the time domain through impulsive stimulated Raman scattering

Femtosecond time-resolved observations of coherent molecular vibrations are carried out through impulse-stimulated Raman scattering (ISRS). Individual cycles of vibrational oscillation, as well as vibrational dephasing, are time resolved. ISRS therefore provides a means through which time-resolved spectroscopy of vibrationally distorted molecules can be carried out. Several experimental configurations involving either crossed excitation pulses or a single excitation pulse are discussed theoretically and demonstrated experimentally. >

Ultrafast photodissociation of I3− in solution: Direct observation of coherent product vibrations

We report the observation of terahertz oscillations in transient transmission measurements following femtosecond UV photolysis of I−3 in ethanol solutions. Transmission signals at 620 nm and at 880 nm, which are above and below the λmax of the known absorption of I−2 oscillate at precisely opposite phase. This and other results presented indicate that though the oscillations we are observing coherent vibration of the I−2 photofragment.

Large amplitude ground state vibrational coherence induced by impulsive absorption in CsI. A computer simulation

Abstract Computer simulations are presented to show that impulsive optical excitation of CsI, in various vibrational levels, can give rise to large-amplitude vibrational coherence in the ground electronic state. It is further demonstrated that using transient absorption spectroscopy, such motion may be directly detected, and that this scheme should be experimentally realizable using currently available laser sources. The quantum mechanical propagation method used in this study, proves to be well suited for the simulation of optical excitation of small molecules, under conditions where the assumptions of time-dependent perturbation theory are violated.

Intermolecular vibrational motion in CS2 liquid at 165 ⩽ T⩾ 300 K observed by femtosecond time-resolved impulsive stimulated scattering

Abstract Temperature-dependent molecular dynamics of CS 2 liquid are examined in femtosecond time-resolved impulsive stimulated scattering experiments. At temperatures T ⩽ 240 K, weakly oscillatory time-dependent responses are observed. These are interpreted in terms of librations of molecules about their local potential minima. The librations undergo rapid inhomogeneous dephasing. Temperature-dependent values of the configuration-averaged librational frequency and the extent of inhomogeneity in that frequency are determined.

Ultrafast photodissociation of I3− in ethanol: A molecular dynamics study

A detailed molecular model of I3− photodissociation in liquid ethanol is developed. Extensive molecular dynamics trajectory calculations are used to determine product energy distribution, time‐dependent spectra, and reorientation dynamics in semiquantitative agreement with experimental data.

Molecular dynamics in liquids from femtosecond time-resolved impulsive stimulated scattering

Molecular motions in simple liquids are observed on the femtosecond time scale by means of impulsive simulated scattering. In carbon disulphide and benzene liquids, weakly oscillatory time-dependent responses are observed. These are interpreted in terms of molecular orientational motion which at short times is vibrational (i.e. librational) in character due to intermolecular interactions. In CS/sub 2/, the dephasing of the oscillations is primarily inhomogeneous. Temperature-dependent values of the configuration-averaged librational frequency and the extent of its inhomogeneity are determined. >