High-resolution resonance-enhanced multiphoton photoelectron circular dichroism

Abstract

Photoelectron circular dichroism (PECD) is a highly sensitive enantiospecific spectroscopy for studying chiral molecules in the gas phase using either single-photon ionization or multiphoton ionization. In the short pulse limit investigated with femtosecond lasers, resonance-enhanced multiphoton ionization (REMPI) is rather instantaneous and typically occurs simultaneously via more than one vibrational or electronic intermediate state due to limited frequency resolution. In contrast, vibrational resolution in the REMPI spectrum can be achieved using nanosecond lasers. In this work, we follow the high-resolution approach using a tunable narrow-band nanosecond laser to measure REMPI-PECD through distinct vibrational levels in the intermediate 3s and 3p Rydberg states of fenchone. We observe the PECD to be essentially independent of the vibrational level. This behaviour of the chiral sensitivity may pave the way for enantiomer specific molecular identification in multi-component mixtures: one can specifically excite a sharp, vibrationally resolved transition of a distinct molecule to distinguish different chiral species in mixtures.

Experimental and Theoretical Investigation of the 3sp(d) Rydberg States of Fenchone by Polarized Laser Resonance‐Enhanced‐Multiphoton‐Ionization and Fourier Transform VUV Absorption Spectroscopy

“…Since the laser pulse duration is 1.3 ps we may infer that τ is of similar magnitude. This is, of course, quite consistent with the above deduction of a 4 fs lower bound based on the fs laser result, and with the alternative estimate of τ 3s ≥0.6 ps based upon observed ns laser REMPI linewidths [16,19] . It is also fully consistent with a direct lifetime measurement [22] of τ 3s =3.3 ps made by a delayed pump‐probe experiment at 201 nm (single photon resonance).…”

“…Back‐extrapolating, the line of this ridge intersects the line of the adiabatic I. E. at a point 2φ=6.4 eV, which value we have already identified as being indicative of the 3 p thresholds. Similar 3 s and (weak) 3 p ridges are observed in the ns laser REMPI PES although these measurements extend to only just above the 3 p threshold region [19] . In the present results, it can be seen that the initial ridge associated with the Δ v =0 ionization of the 3 s Rydberg state apparently continues throughout the 3 p excitation band.…”

“…For the 3 s simulation we first note an excellent agreement with experiment for the first few tenths eV, and which remains good up to the 3 p onset. Figure 11 demonstrates that this same 3s simulation, but using a reduced peak width parameter to broaden the stick spectrum, also produces excellent agreement with a much higher resolved nanosecond laser REMPI spectrum [29] around the 3 s origin, and so is consistent also with the CC2 simulation for the 3 s Rydberg state that appears in Reference [19] .…”

“…These regimes all contrast greatly with the reported nanosecond REMPI mass spectrum, which shows very extensive fragmentation into multiple fragment channels, predominantly those with mass <40 amu, even at the 3s origin [19] . This degree of fragmentation into the smallest fragment ions even exceeds that seen in the 70 eV electron impact mass spectrum [62] indicating that, energetically, very large numbers of photons must be absorbed.…”

“…These include cavity ring‐down polarimetry, [10] and photoelectron circular dichroism (PECD) measured with synchrotron radiation in both soft X‐Ray C 1s ionization [11] and vacuum ultraviolet (VUV) valence band ionization [12,13] . Even more recently, fenchone has been a primary molecule of choice in fundamental investigations of laser multiphoton ionization PECD [14–20] including pump‐probe time‐resolved measurements [21,22] . Complementing the fundamental laser‐based studies, multiphoton ion yield [23] and enantiomeric excess determination [24,25] experiments have addressed more analytical applications using fenchone as a testcase (Scheme 1).…”