Impulsive Laser Induced Alignment of Molecules Dissolved in Helium Nanodroplets

We show that a 450 fs nonresonant, moderately intense, linearly polarized laser pulse can induce field-free molecular axis alignment of methyliodide molecules dissolved in a helium nanodroplet. Time-resolved measurements reveal rotational dynamics much slower than that of isolated molecules and, surprisingly, complete absence of the sharp transient alignment recurrences characteristic of gas phase molecules. Our results presage a range of new opportunities for exploring both molecular dynamics in a dissipative environment and the properties of He nanodroplets.Comment: 5 pages, 4 figure

Alignment enhancement of molecules embedded in helium nanodroplets by multiple laser pulses

We show experimentally that field-free one-dimensional (1D) alignment of 1,4-diiodobenzene molecules embedded in helium nanodroplets, induced by a single, linearly polarized 200-fs laser pulse, can be significantly enhanced by using two or four optimally synchronized laser pulses. The strongest degree of 1D alignment is obtained with four pulses and gives ⟨cos2θ⟩>0.60. Besides the immediate implications for molecular frame studies, our results pave the way for more general manipulation of rotational motion of molecules in He droplets

Perturbations of electronic transitions of organic molecules in helium droplets generated with a new pulsed droplet source

Within this work the Even-Lavie valve was successfully employed as a new pulsed helium droplet source and was used for the electronic spectroscopy of various organic molecules doped into the droplets. The detailed characterization of the pulsed helium droplet beam revealed a significantly different dependence of the droplet size on the stagnation conditions as compared to a continuous helium droplet beam. The combined investigation of Rayleigh-scattering from the droplets and laser induced fluorescence (LIF) from molecules doped into the droplets revealed a bimodal droplet size distribution. LIF from dopant molecules inside helium droplets could only be observed from the leading fraction of the droplet pulses. The size of these droplets can be varied between 10000 and 1000000 helium atoms per droplet. The other fraction of the droplet pulses carries very large droplets not useful for doping with molecules. Variations of the stagnation conditions mainly affect the relative abundance of the two fractions. The optimum signal is almost constant from single shot up to 500 Hz operation and the density of the droplets is about 20 times higher than in the continuous droplet beam. The electronic spectra of the anthracene-derivatives Anthracene, 9,10-Dichloroanthracene, 9-Chloroanthracene, 9-Cyanonanthracene, 9-Phenylanthracene, 9-Methylanthracene, 1-Methylanthracene and 2-Methylanthracene, the charge transfer molecules Phenylpyrrole and Fluorazene, and the Pyrromethene Dyes BDP, 8-PhPM, PM546, PM567, and PM650, doped into the pulsed helium droplet beam were recorded and compared to the corresponding spectra of the isolated molecules in a supersonic jet. Thereby, significant differences which could not be reduced to the lower temperature in the helium droplets, and which are consequently attributed to the interaction between the helium environment and the embedded molecules, could be observed. The perturbations of the electronic transitions are reflected by substantial line broadening, the occurrence of a fine structure of electronic transitions (zero phonon lines accompanied by phonon wings), and in rare cases a multiplet splitting of zero phonon lines. The broadening of molecular transitions is counterintuitive to what is expected for sub-Kelvin temperature and was attributed to a damping of the electronically excited state by the helium environment which occurs if the electronic excitation induces a significant nuclear rearrangement. Broad electronic spectra could also be observed due to the dominance of phonon wings over pure molecular transitions. Both broadening effects reflect a strong perturbation of the electronic transitions of the dopant species by the helium droplet. This result deduced from a larger systematic investigation is of importance for planning photochemical experiments in helium droplets and contributes to the understanding of the solvation of molecules in superfluid helium droplets

New method to study ion-molecule reactions at low temperatures and application to the H2+_2^+ + H2_2 →\rightarrow H3+_3^+ + H reaction

Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the ${\rm H}_2^+ + {\rm H}_2\rightarrow {\rm H}_3^+ + {\rm H}$ reaction with the ${\rm H}_2^+$ ion prepared in a single rovibrational state at collision energies in the range $E_{\rm col}/k_{\rm B} = 5$-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H$_2$ molecules and the second is a supersonic beam of H$_2$ molecules excited to Rydberg-Stark states of principal quantum number $n$ selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10$^\circ$. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state ${\rm H}_2$ beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below $E_{\rm col}/k_{\rm B}= 10$ K, i.e., below $E_{\rm col}\approx 1$ meV, with an energy resolution of 100 $\mu$eV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical-Langevin-capture model in the collision-energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studyingComment: 39 pages, 10 figure

Microsolvation in superfluid helium droplets studied by the electronic spectra of six porphyrin derivatives and one chlorine compound

After almost two decades of high resolution molecular spectroscopy in superfluid helium droplets, the understanding of microsolvation is still the subject of intense experimental and theoretical research. According to the published spectroscopic work including microwave, infrared, and electronic spectroscopy, the latter appears to be particularly promising to study microsolvation because of the appearance of pure molecular transitions and spectrally separated phonon wings. Instead of studying the very details of the influence of the helium environment for one particular dopant molecule as previously done for phthalocyanine, the present study compares electronic spectra of a series of non-polar porphyrin derivatives when doped into helium droplets consisting of 104–105 helium atoms. Thereby, we focus on the helium-induced fine structure, as revealed most clearly at the corresponding electronic origin. The interpretation and the assignment of particular features obtained in the fluorescence excitation spectra are based on additional investigations of dispersed emission spectra and of the saturation behavior. Besides many dopant-specific results, the experimental study provides strong evidence for a particular triple peak feature representing the characteristic signature of helium solvation for all seven related dopant species

Electronic spectroscopy of 9,10-dichloroanthracene inside helium droplets

The spectroscopy of molecules doped into superfluid helium droplets provides information on both, the dopant molecule and the helium environment. Electronic spectra of 9,10-dichloroanthracene in helium droplets are presented and compared with corresponding gas phase spectra to unravel the influence of the helium environment. The combined investigation of fluorescence excitation and dispersed emission provides information on dynamic processes in addition to energetic conditions. For vibronic states, the helium induced decay channels dominate over all intramolecular channels that contribute to the gas phase behavior. In addition to the triplet splitting caused by the Cl isotopes, a fine structure resolved for all transitions in the fluorescence excitation spectrum was found, which is the signature of microsolvation of this compound in helium droplets. This fine structure is identified as a single pure molecular transition accompanied by a sharply structured phonon wing. The corresponding fine structure measured for bare anthracene shows remarkable differences

Microsolvation of anthracene inside superfluid helium nanodroplets

This article reports on the microsolvation of anthracene in superfluid helium nanodroplets as revealed by electronic spectroscopy. Among the polyacene molecules benzene, naphthalene, anthracene, tetracene and pentacene only anthracene and tetracene have been found to exhibit multiplet splitting of the electronic and vibronic transitions which can not be explained by a rotational fine structure. The experimental approach for the investigation of the multiplet is the combined investigation of the fluorescence excitation spectrum and dispersed emission spectra. New experimental data are presented on the microsolvation of anthracene in helium droplets. A detailed analysis of the anthracene data will be contrasted to corresponding data on tetracene. These data together with those reported for benzene, naphthalene and pentacene might serve to test and develop theoretical models which are needed to understand microsolvation of single molecules in superfluid helium droplets