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Excited-state proton transfer dynamics are reported for the phenol(NH3)n cluster system. Excited-state proton transfer is shown to occur for this system by isotopic substitution of the hydroxyl proton and ammonia protons. The observed dynamics slow from 80 to 600 ps upon cluster deuteration. The effects of cluster vibrational energy and ammonia concentration in the expansion gas are also studied. The results of these experiments are compared with a previous study [Syage and Steadman, J. Chem. Phys. 95, 2497 (1991)] which used a single excitation energy and varied ionization energy. Our experiments indicate that a significant amount of cluster ion fragmentation occurs for this system and caution must be exercised in assigning the decays observed in a given ion mass channel to a particular parent cluster. Due to the difficulties of associating a given decay curve with a cluster of particular mass, a previously proposed model of proton transfer in isolated clusters [Hineman et al., J. Chem. Phys. 97, 3341 (1992)] which details the effect of cluster vibrational energy on the proton transfer rate cannot be applied to the phenol/ammonia system. [ABSTRACT FROM AUTHOR]

Intracluster vibrational redistribution (IVR) and vibrational predissociation (VP) dynamics of 4-ethylaniline (Ar)1, (N2)1, and (CH4)1 clusters have been studied by time-correlated single photon counting, mass-resolved excitation spectroscopy, and dispersed emission spectroscopy. The 4-ethylaniline molecule has a low frequency ethyl group torsion vibrational mode, which is similar in energy to the van der Waals modes of the clusters. This mode, because of its low energy (∼35 cm-1), plays a role in the vibrational dynamics of the clusters studied. The cluster dissociation rates and product state distributions can be modeled by a serial IVR/VP mechanism for which the VP step is treated by the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The resulting agreement between the calculated and experimental rates and product state intensities indicates that a statistical distribution of energy among all low frequency modes exists for 4-ethylaniline/polyatomic solvent clusters in which kIVR>kVP. For 4-ethylaniline (Ar)1 clusters kVP>=kIVR and a statistical distribution of energy among the chromophore and van der Waals modes is not achieved. The central determining factor for the vibrational dynamics of these clusters is overall density of low energy modes. [ABSTRACT FROM AUTHOR]

High resolution stimulated Raman spectroscopy (SRS) has been used to examine N2 and N2/He free jet expansions and also equilibrium samples of N2 from 15 to 110 K. The jet spectra show the formation of large liquid clusters which supercool and subsequently freeze to form crystalline β-N2 solid and, in He expansions, undergo a further transformation to a partially annealed α-N2 form. CW-SRS frequency and linewidth data obtained for equilibrium samples of the condensed phases of N2 yielded frequency–temperature relations used in deducing internal temperatures for the clusters produced in the expansion experiments. Analysis of the cooling curves indicates a mean cluster diameter of 35 nm and favors a prompt freezing process rather than a gradual conversion of liquid to solid in a single cluster on the microsecond time scale of the experiments. Supercooling limits of 34 to 44 K are deduced for the liquid, far below the triple point temperature of 63.2 K at which equilibrium samples freeze. Some evidence for surface versus bulk contributions to the spectra is seen in the asymmetric line shapes observed for liquid clusters in the condensation region. The results show that the high spectral and spatial resolution of nonlinear Raman methods such as SRS and CARS provide a unique probe of the condensation processes in free jet expansions. [ABSTRACT FROM AUTHOR]

Photothermal lensing effects are observed in the early expansion stages (X/D=1–7) of a supersonic free jet and narrow P and R branch rotational lines have been recorded at 100 MHz resolution for the ν2 +5ν3 band of acetylene. Rotational Raman loss spectra were also recorded and, from the collision-broadened linewidths of these, it is estimated that about 100 collisions occur in the beam transit from pump to probe positions. This is sufficient to produce rotational and some vibrational relaxation in forming the lens. The photothermal intensities are consistent with a Boltzmann rotational distribution in the ground state and a rotational temperature of 70 K is deduced for 100 PSI expansions of 20% C2 H2 in helium. Charac- teristics of the photothermal signal in the jet are described and the time and spatial dependence of the signal is used to measure a flow velocity 625 m/s for the jet. This photothermal method may prove useful as a diagnostic probe of beam properties and as a spectroscopic measure of weak one- and two-photon transitions of molecules cooled in jets. [ABSTRACT FROM AUTHOR]

Time-resolved stimulated emission pumping has been used to measure the vibrational energy redistribution rate in jet cooled aniline (N2)1 clusters, following 11 excitation. An IVR time of 200±50 ps is obtained, which is consistent with earlier time correlated single photon converting measurements. [ABSTRACT FROM AUTHOR]

Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N2 the photoionization step was via a 2+2 resonantly enhanced multiphoton ionization process using the a1Πg(v=2)←X1Σg(v=1) two-photon resonance. The detection limit is 103–104 times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1+1 ionization step permitted measurements at state densities less than 1010 cm-3 and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active ν16 fundamental. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]