Anomalous light-induced drift of linear molecules

The sudden approximation in energy is used to derive analytic formulas that describe the anomalous light-induced drift (LID) of linear molecules absor bing radiation in the rovibrational transition nJ(i)-mJ(f) (n and m are the ground and excited vibrational states, and J(alpha) is the rotational quan tum number in the vibrational state alpha = m,n). It is shown that for all linear molecules with moderate values B less than or similar to 1 cm(-1) of the rotational constant, anomalous LID can always by observed under the pr oper experimental conditions; temperature T, rotational quantum number J(i) , and type of transition (P or R). The parameter gamma=B[J(i)(J(i)+1)-J(f)( J(f)+1)]nu(n)/2k(B)T(nu(m)-nu(n)) is used to derive a condition for observi ng anomalous LID: gamma similar to 1(k(B) is the Boltzmann constant and nu( alpha) is the transport rate of collisions of molecules in the vibrational state alpha and buffer particles at moderate molecular velocities nu much l ess than<(nu)over bar>(b), where nu<(nu)over bar>(b) is the most probable v elocity of the buffer particles). For nu(m)>nu(n) anomalous LID can be obse rved only in P-transitions, while for nu(m)<nu(n) it can be observed only i n R-transitions. It is shown that anomalous LID is possible for all ratios beta=M-b/M of the masses of the buffer particles (M-b) and of the resonant particles (M) and any absorption-line broadening (Doppler or homogeneous). The optimum conditions for observing anomalous LID are realized when the ab sorption line is Doppler-broadened in an atmosphere of medium-weight (beta similar to 1) and heavy (beta much greater than 1) buffer particles. In thi s case, anomalous LID can be observed in the same transition within a broad temperature interval Delta T similar to T. If the buffer particles are lig ht (beta much less than 1) or if the broadening of the absorption line is h omogeneous, anomalous LID in the same transition can be observed only withi n a narrow temperature range Delta T much less than T. (C) 1999 American In stitute of Physics. [S1063-7761(99)01005-7].