Crossover from the Ultracold to the Quasiclassical Regime in State-Selected Photodissociation

Abstract

Processes that break molecular bonds are typically observed with molecules occupying a mixture of quantum states and successfully described with quasiclassical models, while a few studies have explored the distinctly quantum mechanical low-energy regime. Here we use photodissociation of diatomic strontium molecules to demonstrate the crossover from the ultracold, quantum regime where the photofragment angular distributions strongly depend on the kinetic energy, to the energyindependent quasiclassical regime. Using time-of-flight velocity map imaging for photodissociation channels with millikelvin reaction barriers, we explore photofragment energies in the 0.1-300 mK range experimentally and up to 3 K theoretically, and discuss the energy scale at which the crossover occurs. Furthermore, we find that the effects of quantum statistics can unexpectedly persist to high photodissociation energies.

Experimental and theoretical investigation of the crossover from the ultracold to the quasiclassical regime of photodissociation

“…(5) cancel for all energies [25]. Our experimental measurements of angular distributions always agree with quantum mechanical calculations [17]. For photodissociation of molecules in analogous quantum states but composed of identical fermionic isotopes we expect to observe differ-ent distributions than in Fig.…”

“…Quasiclassical and quantum mechanical calculations of the photofragment angular distributions do not generally converge in the axial recoil limit if the photofragments are identical particles. Experimental measurements always agree with the quantum theory[17]. Here the high-energy angular distributions are calculated with the quasiclassical(6)and quantum mechanical (3) models for the 1u initial states with (a) Ji = 1, Mi = {0, 1}, P = 1; (b) Ji = 3, Mi = {1, 2}, P = 0; and (c) Ji = 3, Mi = {0, 1, 2, 3}, P = 1.…”

“…The experiment can sample energies from ∼ 0.1 mK limited by the molecule trap depth to ∼ 100 mK limited by the available laser intensity that is needed to overcome the diminishing tran-sition strengths. The rotational and electronic potential barriers that are explored in this and related [17] work fall within this range, permitting direct observations of the crossover. We reduce the process to a single initial molecular state and two angular-momentum product channels, in which case the quantum mechanics of the reaction can be encoded in a single pair of coefficients: the amplitude and phase of the channel interference.…”

“…The photodissociation cross sections modified by spin statistics are not correctly described by the quasiclassical model (6), even in the axial recoil limit of large continuum energies. This effect is observable in experiments as a deviation of high-energy photodissociation images from the quasiclassical model for certain initial molecular states and light polarizations [17]. Figure 6 illustrates 88 Sr 2 photodissociation pathways where photofragment angular distributions in the axial recoil limit (3) cannot be described by the quasiclassical model (6).…”

“…A broad imaging beam, resonant with a strong 461 nm transition in atomic Sr, nearly copropagates with the lattice and is directed at a chargecoupled device camera that collects absorption images of the photofragments. A typical experiment uses ∼ 10 µs long photodissociation laser pulses, ∼ 100 µs of free expansion, and ∼ 10 µs absorption imaging pulses [17]. Several hundred images are averaged in each experiment.…”