Effect of hyperfine depolarization upon creation and detection of alignment in free-jet expansions via selective photodissociation

Abstract

The creation of alignment by photodissociation is a well-accepted process. When an isotropic sample of gas is subjected to a strong linearly polarized laser pulse at a frequency at which the photodissociation cross section is large, the surviving molecules are usually aligned. If the transition is parallel, μ lies along the internuclear axis (ΔΛ=0) and the surviving molecules will be peaked around M=0, while for a perpendicular transition (ΔΛ=1) the surviving molecules will be peak around M=J. Although this effect has been seen in laser cavities and in the focus of laser beams, it has not been used to create aligned pulses of gas from free-jet expansions. We present the theoretical calculations for the practical creation of alignment in short free-jet gas pulses via saturation photodissociation. Our methodology allows the propagation of the laser light along any direction and with any polarization, the quantification of the effect of hyperfine and electron spin depolarization upon the creation of alignment, direct comparison of the degree of alignment created in parallel versus perpendicular transitions when the polarizations are set to cause M=J versus M=0 peaked distributions, experimental determination of the degree of alignment after depolarization using a second fixed frequency laser, and experimental determination of the degree of alignment prior to depolarization without the use of an additional laser. Our calculations show that hyperfine and electron spin depolarization are the limiting forces in the creation of aligned pulses of gas. These effects are most pernicious in free-jet expansions where only the lowest rotational states are populated and therefore even modest values of nuclear spin and electron spin can effect large depolarizations. The calculations show that these depolarization effects can be effectively mitigated by three methods: (1) using molecules with small Be values, (2) limiting the free-jet expansion so the rotational temperature is above about 50 K, and (3) employing parallel photodissociation transitions (ΔΛ=0). For very cold expansions, parallel transitions are less susceptible to depolarization than perpendicular transitions (ΔΛ=1) because in parallel transitions, the low J states are selectively photodissociated.