Laser Mass Spectrometry with Circularly Polarized Light: Circular Dichroism of Cold Molecules in a Supersonic Gas Beam

An experiment on chiral molecules that combines circular dichroism (CD) spectroscopy, mass-selective detection by laser mass spectrometry (MS), and cooling of molecules by using a supersonic beam is presented. The combination of the former two techniques (CD-laser-MS) is a new method to investigate chiral molecules and is now used by several research groups. Cooling in a supersonic beam supplies a substantial increase in spectroscopic resolution, a feature that has not yet been used in CD spectroscopy. In the experiments reported herein, a large variation in the electronic CD of carbonyl 3-methylcyclopentanone was observed depending on the excited vibrational modes in the n→π∗ transition. This finding should be of interest for the detection of chiral molecules and for the theoretical understanding of the CD of vibronic bands. It is expected that this effect will show up in other chiral carbonyls because the n→π∗ transition is typical for the carbonyl group. Keep it cool! Cooling of a chiral carbonyl in a supersonic beam (T _rot≈15 K) results in unexpectedly large variations in the circular dichroism (CD; see figure). An increase in the anisotropy factor of up to g>0.4 is observed. This substantially exceeds the increase in the CD effect measured in (1+1+1)-multiphoton ionization experiments. Other bands show a significant CD reduction, which is an indication of the strong influence of vibrations on the electronic CD at λ=325 nm.