Purcell, S.M.; (2010) Laser induced molecular motion in strong nonresonant laser fields. Doctoral thesis , UCL (University College London).

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Abstract

The optical dipole force from a singe focussed laser beam was used to study the role of laser-induced molecular alignment on the centre-of-mass motion of carbon disulphide molecules in a molecular beam. The translational, rotational and vibrational temperatures of the carbon disulphide molecules were measured to be 3.4\pm0.2 K, 35\pm10 K and 250\pm14 K respectively. The velocity of the beam was measured to be 542\pm22 m s ^{-1}. Time-of-flight mass spectroscopy was used to measure the acceleration and deceleration of the molecules. Maximum velocity changes of 7.5 m s ^{-1} and 10 m s ^{-1} were recorded for linearly and circularly polarised light respectively. These results showed that the dipole force, \digamma \alpha \bigtriangledown [\alpha_e_f_f(I)I(r)], where \alpha_e_f_f is the effective polarisability and determined through laser-induced alignment, can be modified by changing the laser polarisation. For linearly and circularly polarised light, a 12% difference in effective polarisability was measured to produce a 20% difference in dipole force. The dipole force from a single focussed laser beam produces a molecular optical lens and the downstream density of the molecular focus was probed by measuring the ion signal for both laser polarisations. The focal lengths for linearly and circularly polarised light were found to be separated by \approx 100 \mu m. By altering the laser polarisation from linearly through elliptically to circularly polarised light, the focal length of the molecular optical lens could be smoothly altered over the \approx 100 \mu m focal range. The role of the effective polarisability of each rotational state was also studied numerically. Separate rotational states were found to significantly alter the focal properties of a molecular optical lens. In carbon disulphide, higher rotational states (J > 10), exhibit less molecular alignment and when occupied, the focal length of the molecular optical lens for these states was increased by 60 % compared to the ground state.

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