Quantitative study of enantiomer-specific state transfer

Abstract

We here report on a quantitative study of Enantiomer-Specific State Transfer (ESST), performed in a pulsed, supersonic molecular beam. The chiral molecule 1-indanol is cooled to low rotational temperatures (1-2 K) and a selected rotational level in the electronic and vibrational ground state of the most abundant conformer is depleted via optical pumping on the S1 ← S0 transition. Further downstream, three consecutive microwave pulses with mutually perpendicular polarizations and with a well-defined duration and phase are applied. The population in the originally depleted rotational level is subsequently monitored via laser induced fluorescence (LIF) detection. This scheme enables a quantitative comparison of experiment and theory for the transfer efficiency in what is the simplest ESST triangle for any chiral molecule, that is, the one involving the absolute ground state level, |JK a Kc = |000 . Moreover, this scheme improves the enantiomer enrichment by over an order of magnitude compared to previous works. Starting with a racemic mixture, a straightforward extension of this scheme allows to create a molecular beam with an enantiomer-pure rotational level, holding great prospects for future spectroscopic and scattering studies.

Rational Pulse Design for Enantiomer-Selective Microwave Three-Wave Mixing

“…A recent three-wave mixing experiment [17] has followed a different path to circumvent loss of efficiency due to both thermal population in the excited rotational states and orientational degeneracy: The experiment addresses rotational levels with J = 0 and J = 1 with the excited rotational levels depleted prior to the three-wave mixing. Of course, in a typical molecular sample, only a small amount of molecules resides initially in the rotational ground state.…”

“…In view of enantiomer separation with electric fields only, it will therefore be important to combine the optical depletion technique of Ref. [17] with the pulse design presented here, in order to sequentially carry out several three-wave mixing cycles and thereby address a larger set of rotational states.…”

“…7, whereas levels 3 13 and 3 12 are empty. Such an initial condition can be realized by choosing the two excited rotational states in a vibrational state ν > 0 [24,26] or by depleting excited rotational levels by laser excitation [17]. Figures 8 and 9 show the population transfer for different pulse sequences, with the average population of the three rotational levels for the two enantiomers plotted in Fig.…”

“…These techniques are based on light-matter interaction in the dipole approximation, where the enantiomer-selective observable arises as a triple product of molecule-specific vectors which changes sign under exchange of the two enantiomers, independent of the molecular orientation [12,13]. In addition to detecting enantiomeric excess, microwave three-wave mixing (3WM) can also be used to selectively excite enantiomers to different energy levels [14][15][16][17]. This can serve as a precursor for the separation of enantiomers and the preparation of an enantio-pure sample out of a racematic mixture [18,19].…”

“…The problem of temperature can be solved by addressing levels which are sufficiently excited such that their thermal population vanishes [24,26]. Alternatively, the thermal population in the excited levels can be eliminated prior to the three-wave mixing, for example by optical pumping of an electronic transition [17]. The limitation due to the orientational degeneracy can also be mitigated-it requires a sufficiently large number of electric fields that break the corresponding symmetry [27].…”