Theoretical description of adiabatic laser alignment and mixed-field orientation: the need for a non-adiabatic model

We present a theoretical study of recent laser-alignment and mixed-field-orientation experiments of asymmetric top molecules. In these experiments, pendular states were created using linearly polarized strong ac electric fields from pulsed lasers in combination with weak electrostatic fields. We compare the outcome of our calculations with experimental results obtained for the prototypical large molecule benzonitrile (C$_7$H$_5$N) [J.L. Hansen et al, Phys. Rev. A, 83, 023406 (2011)] and explore the directional properties of the molecular ensemble for several field configurations, i.e., for various field strengths and angles between ac and dc fields. For perpendicular fields one obtains pure alignment, which is well reproduced by the simulations. For tilted fields, we show that a fully adiabatic description of the process does not reproduce the experimentally observed orientation, and it is mandatory to use a diabatic model for population transfer between rotational states. We develop such a model and compare its outcome to the experimental data confirming the importance of non-adiabatic processes in the field-dressed molecular dynamics.Comment: 11 pages, 9 figure

Photodissoziation von Halogenwasserstoffen in und auf Edelgasclustern

The object of this work are experiments on the photodissociation of hydrogen halides (HX) on the surface and embedded inside of rare gas clusters. The aim of the experiments was to explore the cage effect on the H atoms caused by the rare gas clusters. Pulses from a laser system operating at a wavelength of 243 nm were used to dissociate the HX molecules and to ionize the resulting H fragments in a (2+1) REMPI process. Flighttimes of the H ions were measured using a time-of-flight mass spectrometer operating in the so called low-field-mode. An existing trajectory simulation program was developed into a program calculating the H fragment kinetic energy distributions (HKED) based on the accompanying flighttime spectra. The mixed clusters were formed in two ways: First, producing pure rare gas clusters and using the pickup technique to prepare HX molecules on the surface of the clusters with a mean rare gas cluster size about anti N#approx#130. Second, argon clusters with HBr embedded inside are achieved by the seeded-beam technique expanding a dilute mixture of HBr in Ar. The resulting HKED for all configurations show a strong cage effect. In the 'surface' configuration different results were received: With increasing atom mass of the rare gases the energy loss of the H fragments decreases. Comparing the hydrogen halides more H fragments are produced with total energy loss in case of HBr than in case of HI. The HKED measured for the embedded configuration yield the strongest cage effect in this measurements. A variation of the cluster size was carried out with a maximum at a mean cluster size of anti N=115. As result a decreasing energy loss was detected at lower mean cluster sizes anti N. (orig.)Available from TIB Hannover: RA 1396(1999/12) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Photodissociation of HBr molecules and clusters: Anisotropy parameters, branching ratios, and kinetic energy distributions

The ultraviolet photolysis of HBr molecules and (HBr)n clusters with average size around = 9 is studied at three different wavelengths of 243, 205, and 193 nm. Applying polarized laser light, the kinetic energy distribution of the hydrogen photofragment is measured with a time-of-flight mass spectrometer with low extraction fields. In the case of HBr monomers and at 243.1 nm, an almost pure perpendicular character (= –0.96 ± 0.05) of the transitions is observed leading to the spin–orbit state Br(2P3/2). The dissociation channel associated with the excited state Br*(2P1/2) is populated by a parallel transition (* = 1.96 ± 0.05) with a branching ratio of R = 0.20 ± 0.03. At the wavelength of 193 nm, about the same value of R = 0.18 ± 0.03 is found, but both channels show a mainly perpendicular character with = –0.90 ± 0.10 for Br and * = 0.00 ± 0.10 for Br*. The results for 205 nm are in between these two cases. For the clusters at 243 nm, essentially three different groups appear which can be classified according to their kinetic energy: (i) A fast one with a very similar behavior as the monomers, (ii) a faster one which is caused by vibrationally and rotationally excited HBr molecules within the cluster, and (iii) a slower one with a shoulder close to the fast peak which gradually decreases and ends with a peak at zero velocity. The zero energy fragments are attributed to completely caged H atoms. The angular dependence of the group (iii) is isotropic, while that of the other two is anisotropic similar to the monomers. At 193 nm only the fast and the slow part is observed without the peak at zero energy. Apparently the kinetic energy is too large to be completely dissipated in the cluster

Photodissociation of oriented HXeI molecules generated from HI-Xe_n clusters

We report the production in the gas phase of ionically bound HXeI molecules. The molecules are generated by the photodissociation of HI molecules in large Xen clusters and are identified from the asymmetry of the detected H atom fragments arising from the dissociation of oriented HXeI. The orientation, resulting from a synergistic action of a pulsed laser field with a weak electrostatic field, is quite pronounced, due to a large ratio of the polarizability anisotropy to the rotational constant of HXeI

Photodissociation of HBr in and on Ar_n clusters: the role of the position of the molecule

Photodissociation experiments are carried out for single HBr molecules which are embedded in the interior or absorbed on the surface of large Arn clusters. For the embedded case the size dependence is measured for the average sizes n=51, 139, 230, 290 and 450. For the surface case and the average size n=139 the source temperature is varied from T=163 K to 263 K. The measured kinetic energy of the H atom fragments exhibits peaks at zero and 1.3 eV which mark completely caged and unperturbed fragments, respectively. The results are compared with Molecular Dynamics simulations which account for the quantum librational delocalization of the HBr molecule. The location of the molecule in/on the cluster is obtained from a trajectory study of the pick-up process under realistic conditions. For the embedded case corresponding to a co-expansion experiment, three argon layers are sufficient to completely hinder the H atom, in perfect agreement with the calculations. For the pick-up experiment, the large change of the source temperature leads to very small changes of cluster temperature dependent properties. Events starting from the second shell have a higher exit probability than those coming from the surface