Wavelength dependent photoelectron circular dichroism of limonene studied by femtosecond multiphoton laser ionization and electron-ion coincidence imaging

Abstract

A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. AbstractEnantiomers of the monoterpene limonene have been investigated by (2+1) resonance enhanced multiphoton ionization and photoelectron circular dichroism employing tuneable, circularly polarized femtosecond laser pulses. Electron imaging detection provides 3D momentum measurement while electron-ion coincidence detection can be used to mass-tag individual electrons. Additional filtering, by accepting only parent ion tagged electrons, can be then used to provide discrimination against higher energy dissociative ionization mechanisms where more than three photons are absorbed to better delineate the two photon resonant, one photon ionization pathway. The promotion of different vibrational levels and, tentatively, different electronic ion core configurations in the intermediate Rydberg states can be achieved with different laser excitation wavelengths (420 nm, 412 nm, and 392 nm), in turn producing different state distributions in the resulting cations. Strong chiral asymmetries in the lab frame photoelectron angular distributions are quantified, and a comparison made with a single photon (synchrotron radiation) measurement at an equivalent photon energy.

Intense Vibronic Modulation of the Chiral Photoelectron Angular Distribution Generated by Photoionization of Limonene Enantiomers with Circularly Polarized Synchrotron Radiation

“…Despite, or because of, its spectroscopic complexity limonene presents an interesting challenge for further investigation. In a complementary study using (2+1) resonance enhanced ionization of limonene by fs laser pulses we were able to show how, by exploiting the Δ v =0 propensity rule for ionization out of a Rydberg intermediate state, it was possible to exert some control over the final vibrational state in the cation and hence to influence the PECD by pumping through different intermediate state levels. These opportunities were at the time constrained by limited tunability and the relatively large bandwidth of the fs laser pulses, but these can be improved upon using longer pulse durations to provide more complete spectroscopic selectivity.…”

“…have measured the gas phase absorption circular dichroism (CD) of limonene in this region and Logé and Boesl have correspondingly measured the CD (in the ion yield) generated by 2 photon laser ionization of R‐limonene enantiomer at 213 nm. Two photon resonant (2+1) laser multiphoton ionization, for λ ≈400 nm, has also been used to investigate the PECD of limonene enantiomers by examining the forward‐backward asymmetry in the photoelectron angular distribution that results . Although nominally exciting through the same intermediate states at the two photon level, the PECD asymmetry is several orders of magnitude greater than the absorption CD owing to the former effect arising in the pure electric dipole approximation .…”

“…The interpretation of the multiphoton (MP‐)PECD remains challenging. Measurements made with 150 fs duration laser pulses were performed at a small number of wavelengths that coincided with resonant excitation of different vibrational levels of the 2π→3s Rydberg intermediate, and with the 1π→3s intermediate . These showed that a reversal of the sign of the MP‐PECD was possible in the latter case.…”

“…Twop hotonr esonant (2 + 1) laser multiphoton ionization,f or l % 400 nm, has also been used to investigate the PECD of limonene enantiomersb ye xaminingt he forward-backward asymmetry in the photoelectron angular distribution that results. [20][21][22] Although nominally exciting through the same intermediate states at the two photon level, the PECD asymmetryi ss everal orders of magnitude greater than the absorption CD owing to the former effecta rising in the pure electric dipole approximation. [23] Such an enhanced asymmetry for VUV PECD, in the 1%-40 %r ange, is by now well established.…”

Spectroscopy for model heterogeneous asymmetric catalysis

“…Accordingly, high sensitivity and possibility of discrimination in a mixture of products are inevitable. To study the ee of gas‐phase sample, CD has been combined with various methods such as angular‐resolved photoelectron spectroscopy and mass spectrometry . A recent review summarizes the majority of the efforts to analyze chirality in the gas phase .…”