Evidence of a shift between one- and two-photon processes associated with benzene trapped in helium nanodroplets

Bimodal velocity and size distributions of pulsed superfluid helium droplet beams

We report detailed measurements of velocities and sizes of superfluid helium droplets produced from an Even-Lavie pulse valve at stagnation pressures of 20-60 atm and temperatures between 5.7 and 18.0 K. By doping neutral droplets with Rhodamine 6G cations produced from an electrospray ionization source and detecting the positively charged droplets at two different locations along the beam path, we determine the velocities of the different groups of droplets. By subjecting the doped droplet beam to a retardation field, size distributions can then be analyzed. We discover that at stagnation temperatures above 8.0 K, a single group of droplets is observed at both locations, but at 8.0 K and below, two different groups of droplets with different velocities are detectable. The slower group, considered from fragmentation of liquid helium, cannot be deterred by the retardation voltage at 9 kV, implying an exceedingly large size. The faster group, considered from condensation of gaseous helium, has a bimodal distribution when the stagnation temperatures are below 12.3 K at 20 and 40 atm, or 16.1 K at 60 atm. We also report similar size measurements using low energy electrons for impact ionization, and this latter method can be used for facile in situ characterization of pulsed droplet beams. The mechanism of the bimodal size distribution of the condensation group and the reason for the coexistence of both the condensation and fragmentation groups remain elusive.

Suppression of Multiphoton Ionization of Aniline in Large Superfluid Helium Droplets

We report suppression of multiphoton ionization (MPI) of aniline doped large superfluid helium droplets containing over 5 × 10⁶ atoms. In contrast, surface-bound sodium atoms and dimers are readily desorbed and ionized. Adequacy of the experimental conditions is also confirmed from ejection of embedded aniline cations from smaller droplets containing multiple cations, and MPI of gaseous aniline. The photoelectrons have a mean-free-path of less than 1 nm and a thermalization distance of 10 nm. In a droplet with a diameter of over 70 nm, effective charge recombination within the droplet is expected.

Multiple photon excitation and ionization of NO in and on helium droplets

The photoexcitation of NO embedded in superfluid Hen nanodroplets having n approximately 10(4) has been examined. Two-photon excitation prepares electronically excited states (NO(*)), most notably, the embedded analog of the A 2Sigma state of gas phase NO. Vertical excitation to this low Rydberg state is blueshifted and broadened relative to its gas phase counterpart because of the repulsive electron-helium interaction. Transport to the droplet surface is believed to be facile in the superfluid. For example, NO* prefers (energetically) to reside at the droplet surface rather than at the droplet center, in contrast to NO. Photoionization of surface-bound NO* occurs over a significant photon energy range. This yields small cluster ions NO+Hek) with approximately 90% of these clusters having k< or =10. The variation of ion yield with photon energy displays a precipitous change in the region of 24 300-24 400 cm(-1) for all values of k. Possible photoionization mechanisms are discussed and it is suggested that intermediate levels with high-n Rydberg character play a role. This work underscores the important role played by transport in the photophysics of species embedded in the superfluid host.

A systematic shift in the electronic spectra of substituted benzene molecules trapped in helium nanodroplets

Electronic spectra (S1<--S0) have been recorded from five separate substituted benzene derivatives trapped in helium nanodroplets. Each member of the series is found to exhibit a blueshift with respect to the equivalent transition in the gas phase. Taken together with previous results for benzene, the observed shifts show a remarkably good correlation with changes in electron density that occur within each of the aromatic rings as a result of electronic excitation.

Femtosecond lasers in gas phase chemistry

This critical review is intended to provide an overview of the state-of-the-art in femtosecond laser technology and recent applications in ultrafast gas phase chemical dynamics. Although "femtochemistry" is not a new subject, there have been some tremendous advances in experimental techniques during the last few years. Time-resolved photoelectron spectroscopy and ultrafast electron diffraction have enabled us to observe molecular dynamics through a wider window. Attosecond laser sources, which have so far only been exploited in atomic physics, have the potential to probe chemical dynamics on an even faster timescale and observe the motions of electrons. Huge progress in pulse shaping and pulse characterisation methodology is paving the way for exciting new advances in the field of coherent control.