Communication: Observation of dipole-bound state and high-resolution photoelectron imaging of cold acetate anions

Abstract

We report the observation of a dipole-bound state and a high-resolution photoelectron imaging study of cryogenically cooled acetate anions (CH3COO−). Both high-resolution non-resonant and resonant photoelectron spectra via the dipole-bound state of CH3COO− are obtained. The binding energy of the dipole-bound state relative to the detachment threshold is determined to be 53 ± 8 cm−1. The electron affinity of the CH3COO neutral radical is measured accurately as 26 236 ± 8 cm−1 (3.2528 ± 0.0010 eV) using high-resolution photoelectron imaging. This accurate electron affinity is validated by observation of autodetachment from two vibrational levels of the dipole-bound state of CH3COO−. Excitation spectra to the dipole-bound states yield rotational profiles, allowing the rotational temperature of the trapped CH3COO− anions to be evaluated.

Observation of Excited Quadrupole-Bound States in Cold Anions

“…Seventeen above-threshold peaks are observed, due to autodetachment from excited vibrational levels of the QBS. The inset shows the simulation of the rotational profile for peak 2 at a high resolution scan, yielding a rotational temperature of 30-35 K, consistent with previous estimates [21,44]. See the Supplemental Material for details of the rotational simulation [40].…”

“…All the major vibrational peaks and their relative intensities agree with each other in the two spectra, except that the photodetachment spectrum has higher resolution. This resemblance provides strong evidence for the weakly bound nature of the excess electron in the QBS, which has little effect to the neutral core, just as that observed for DBSs [20][21][22][23]. Hence, the photodetachment spectrum of the QBS can be used to yield more accurate vibrational information for the neutral radical.…”

“…Excited DBSs of valence-bound anions were first observed as resonances in photodetachment cross sections [16] and studied via rotational and vibrational autodetachment [17][18][19]. Vibrationally induced autodetachment from selected vibrational levels of an excited DBS, yielding highly non-Franck-Condon resonant PE spectra, has also been investigated recently [20][21][22][23].…”

Perspective: Electrospray photoelectron spectroscopy: From multiply-charged anions to ultracold anions

“…The simulation yielded a rotational temperature of T rot = 35 K, while the cryogenic trap was operated at 4.4 K measured off the trap surface using a silicon diode temperature sensor. More recent experiments on the deprotonated thymine anion gave rise to a similar rotational temperature, 157 whereas a slightly lower rotational temperature was observed for the smaller acetate anion (∼20-35 K), 158 which was the lowest rotational temperature achieved thus far in the cryogenic Paul trap. The discrepancy between the nominal trap temperature (4.4 K) and the achieved ion temperature is most likely due to the RF-heating.…”

“…The potentials of resonant PES via DBS have been illustrated in a number of recent studies. [157][158][159] The resonant PES led to the definitive determination of the 0-0 detachment transition in the acetate anion, allowing the EA of the acetyloxyl radical to be accurately and unambiguously measured. 158 Autodetachment spectroscopy and resonant PE images and spectra for the deprotonated thymine and the 2-hydroxyphenoxide anions have also been reported recently, 157,159 allowing the measurements of the vibrational frequencies of the corresponding neutral radicals and further demonstrating the potentials of resonant PES via DBS as a new technique for vibrational spectroscopy of novel radical species.…”