Femtosecond Stimulated Emission Pumping Research Articles

Vibrational relaxation in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters excited by femtosecond stimulated emission pumping

Vibrational relaxation dynamics in I2−(Ar)n (n=1,2,6,9) and I2−(CO2)n (n=1,4,5) clusters are studied using femtosecond stimulated emission pumping (fs-SEP) in conjunction with femtosecond photoelectron spectroscopy. fs-SEP generates coherently excited I2− within the cluster; results are reported here for excitation energies of 0.57 and 0.75 eV. The time-dependent PE spectra track relaxation of the clustered I2− through coherent intensity oscillations observed at short times (<10 ps) and shifts of the photoelectron spectra that can be seen out to several hundred picoseconds. The relaxation rates depend on the cluster type and excitation energy: the overall time scale in I2−(CO2)n clusters is relatively independent of both, but in I2−(Ar)n clusters the time scale generally increases with cluster size and decreases with excitation energy. The observed dynamics for I2−(CO2) and several of the I2−(Ar)n clusters directly probe the time scale for solvent evaporation.

Vibrational relaxation in clusters: Energy transfer in I2−(CO2)4 excited by femtosecond stimulated emission pumping

Vibrational relaxation dynamics in I2−(CO2)4 clusters are monitored by femtosecond stimulated emission pumping in conjunction with femtosecond photoelectron spectroscopy. Femtosecond pump and tunable dump pulses coherently excite the I2− within the cluster with vibrational energies ranging from 0.57 to 0.86 eV; the subsequent dynamics are monitored via the time-dependent photoelectron spectrum, and are compared to those resulting from excitation of bare I2−. Two observables are used to follow the vibrational relaxation from the vibrationally excited I2− to the surrounding solvent molecules. From 0 to 4 ps, relaxation is apparent through a time-dependent increase in the oscillation which is monitored at its inner turning point. At longer times, out to ∼100 ps, shifts in the photoelectron spectra are used to determine the vibrational energy content of the I2−. Indirect evidence is presented for early rapid energy loss during the first half-oscillation of the wave packet across the potential.

Cluster calorimetry by femtosecond stimulated emission pumping: Excitation and evaporative cooling ofI2−(CO2)n

Femtosecond stimulated emission pumping has been employed as a technique to prepare small clusters with well-known amounts of internal excitation. In this way ${\mathrm{I}}_{2}^{\ensuremath{-}}({\mathrm{CO}}_{2}{)}_{4,5}$ clusters are prepared with 0.58 to 0.99 eV of internal energy. Subsequent evaporation of between two and four ${\mathrm{CO}}_{2}$ monomers from the primary cluster is observed as a function of excitation energy using tandem time-of-flight mass spectrometry. Analysis of the appearance energies of the open channels using phase-space theory and photoelectron spectroscopy data yields binding energies of the first eight ${\mathrm{CO}}_{2}$ molecules to ${\mathrm{I}}_{2}^{\ensuremath{-}}({\mathrm{CO}}_{2}{)}_{n}$ clusters.

Erratum: “Femtosecond stimulated emission pumping: Characterization of the I2− ground state” [J. Chem. Phys. 112, 8847 (2000)

Erratum: “Femtosecond stimulated emission pumping: Characterization of the I2− ground state” [J. Chem. Phys. <b>112</b>, 8847 (2000)

Femtosecond stimulated emission pumping:

Abstract Femtosecond stimulated emission pumping in conjunction with femtosecond photoelectron spectroscopy is used to monitor dynamics within I2−(CO2)4 following coherent vibrational excitation of the I2− chromophore. Femtosecond pump and dump pulses create an I2− wavepacket with 0.53 eV vibrational energy. Subsequent evolution of this wavepacket is monitored via its time-dependent photoelectron spectrum. At this energy, the vibrational frequency of the embedded I2− is 80 cm−1, 7 cm−1 higher than that of bare I2−. As the I2− loses energy to the CO2 molecules, the wavepacket frequency increases linearly at a rate of 3.8 cm−1/ps during the initial 3 ps of coherence. The rate of energy transfer can be determined either by this increase in vibrational frequency or by the shift of the measured photoelectron spectrum. No solvent evaporation during the first 7 ps is observed, but three CO2 molecules evaporate on a microsecond time scale.

Femtosecond stimulated emission pumping: Characterization of the I2− ground state

Femtosecond stimulated emission pumping in combination with femtosecond photoelectron spectroscopy is used to characterize the potential energy function of the I2−(X̃ 2Σu+) ground state up to vibrational energies within 2% of the dissociation limit. The frequency and anharmonicity of this state are measured at a series of vibrational energies up to 0.993 eV by coherently populating a superposition of ground state vibrational levels using femtosecond stimulated emission pumping, and monitoring the resulting wave packet oscillations with femtosecond photoelectron spectroscopy. The dissociative I2−(Ã′ 2Πg,1/2) state is used for intermediate population transfer, allowing efficient population transfer to all ground state levels. Using the measured frequencies and anharmonicities, the X̃ 2Σu+ state has been fit to a modified Morse potential with the β-parameter expanded in a Taylor series, and the bond length, well depth, and υ=0–1 fundamental frequency set equal to our previously determined Morse potential [J. Chem. Phys. 107, 7613 (1997)]. At high vibrational energies, the modified potential deviates significantly from the previously determined potential.