  We evaluate the shifts imparted to vibrational and rotational levels of a
linear molecule by a nonresonant laser field at intensities of up to 10^12
W/cm^2. Both types of shift are found to be either positive or negative,
depending on the initial rotational state acted upon by the field. 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 87Rb_2 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 5x10^9
W/cm^2 are capable of dissociating the molecule due to the vibrational shift.
Lesser shifts can be used to fine tune the rovibrational levels and thereby to
affect collisional resonances by the nonresonant light. The energy shifts may
be discernible spectroscopically, at a 10 MHz resolution.
