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Abstract

We evaluate the shifts imparted to vibrational and rotational levels of a linear molecule by a nonresonant laser fi eld at intensities of up to 10¹² W/cm². Both types of shift are found to be either positive or negative, depending on the initial rotational state acted upon by the fi eld. An adiabatic field-molecule interaction imparts a rotational energy shift which is negative and exceeds the concomitant positive vibrational shift by a few orders of magnitude. The rovibrational states are thus pushed downward in such a field. A nonresonant pulsed laser field that interacts nonadiabatically with the molecule is found to impart rotational and vibrational shifts of the same order of magnitude. The nonadiabatic energy transfer occurs most readily at a pulse duration which amounts to about a tenth of the molecule's rotational period, and vanishes when the sudden regime is attained for shorter pulses. We applied our treatment to the much studied ⁸⁷Rb₂ molecule in the last bound vibrational levels of its lowest singlet and triplet electronic states. Our calculations indicate that 15 ns and 1,5 ns laser pulses of an intensity in excess of 5×10⁹ W/cm² are capable of dissociating the molecule, either due to the vibrational shift alone or due to its combination with the rotational shift. Lesser shifts can be used to fi ne tune the rovibrational levels and thereby to aff ect collisional resonances by the nonresonant light. The energy shiftsdue to laser intensities of 10⁹ W/cm² may be discernible spectroscopically, at a 10 MHz resolution.