Alignment of CS2 in intense nanosecond laser fields probed by pulsed gas electron diffraction

Abstract

A pulsed gas electron diffraction apparatus was developed and applied to investigate an alignment process of molecules in intense laser fields. A two-dimensional ͑2D͒ electron diffraction pattern of jet-cooled CS 2 in intense nanosecond laser fields ͑1064 nm, ϳ0.64 TW/cm 2 , 10 ns͒ was measured using short-pulsed 25 keV electron beam packets ͑ϳ7 ns͒ generated by irradiating a tantalum photocathode with the 4th harmonics of pulsed YAG laser light. The observed anisotropic 2D diffraction pattern was analyzed quantitatively by taking into account the spatio-temporal distributions of the laser pulses, the electron beam packets, and the molecular beam through a numerical simulation of the observed diffraction pattern. The anisotropy of the spatial distribution of molecular axes of CS 2 in the laser polarization direction is accounted for by the effect of the intense laser fields. Considering the spatio-temporal averaging effect, the temporal pulse width of an electron packet required for real-time probing of the alignment process of molecules in intense nanosecond laser fields is discussed. A numerical simulation of temporal and spatial profiles of an electron packet is also performed to examine conditions for generating sub-picosecond ultrashort electron pulse for real-time probing of ultrafast molecular dynamics by the pulsed gas electron diffraction method.

Time-Resolved Electron Diffraction from Selectively Aligned Molecules

“…It has been shown theoretically that electron diffraction from aligned molecules in the gas phase reveals not only the interatomic distances but also the corresponding angles [6,7]. Aligning molecules in helium droplets using powerful cw lasers has been proposed [8], and electron diffraction from adiabatically aligned molecules has been demonstrated experimentally [9]. However, the presence of strong alignment fields can affect the structure and dynamics of the molecules under investigation and prevents the study of field-free molecules.…”

Experimental basics for femtosecond electron diffraction studies

“…This eases considerably the sample preparation methods. Indeed, gas-phase electron diffraction is completely free of crystalline order (although laser-alignment can be employed) and yet is still effective [32], even for time-resolved studies [10][11][12]33]. To date, however, the experimental configuration using molecular beams has, owing to velocity mismatch between the laser and electron pulses, limited the overall temporal resolution to the picosecond time-range [34].…”

“…Despite almost three decades of development, the techniques of ultrafast diffraction remain the domain of relatively few laboratories worldwide [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. In the case of ultrafast X-ray diffraction, the primary limitations are the laser systems required for plasma-based sources and the low X-ray flux [18,19] and, in the case of synchrotron-based efforts, scarce beam time and the expense of new facilities.…”

Laser-assisted electron diffraction for femtosecond molecular imaging

“…10 Indeed, this method has been successfully applied to discuss the changes in the geometrical structure of molecules in some photochemical reactions 11,12 and the laserinduced alignment processes of molecules. [13][14][15] However, because of a problem called velocity mismatch effect, 4,16 even when we employ ultimately short electron pulses, the temporal resolution of the GED method could not be better than about one picosecond as long as a gaseous sample is used. While several techniques, such as tilted optical pulse excitation 17 and relativistic electron diffraction, 18,19 have been proposed in order to overcome the limitation in the temporal resolution of GED, a time-resolved GED experiment with a "femtosecond" temporal resolution has yet to be achieved.…”