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The ethyl radical in superfluid helium nanodroplets: Rovibrational spectroscopy and ab initio computations
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Image of FIG. 1.
FIG. 1.

Optimized geometry for ethyl at the CCSD(T)/cc-pVTZ level of theory, as used in the vibrational analysis. Bond lengths are shown in Angstroms and bond angles are shown in degrees.

Image of FIG. 2.
FIG. 2.

Energy level diagram with labelling format, (,,), showing most (e.g., (1,−1,1)←(0,0,1) and (2,−2,1)←(1, + 1,1) are out of range) of the relevant parallel (blue) and perpendicular (red) transitions out of the ground state for each nuclear spin isomer. The parallel transitions correspond to Γ′ = ′ (-type) and the perpendicular transitions correspond to Γ′ = ″ (-type) or ″ (-type). The energy levels were determined using expression (5) in Ref. and the gas phase constants from Ref. . Only states up to and including = = = 2 are included in this figure. The sign of (|| = ) is relative to . For clarity, each state in either = 0 or 1 is labelled once.

Image of FIG. 3.
FIG. 3.

Droplet beam mass spectra with either a cold or hot pyrolysis source and the flow of the di--amyl peroxide precursor adjusted to optimize for the pick-up of single molecules.

Image of FIG. 4.
FIG. 4.

Survey scans of ethyl in helium nanodroplets with different scanning conditions. The dotted lines correspond to the gas phase (sub)band origins. The top scan extends down to 2622 cm and reveals no other depletion signals. These spectra, in addition to difference mass spectra, allowed for the identification of peaks consistent with ethyl, which are indicated by arrows. The features marked by asterisks correspond to known bands from ethylene, ethane, and acetone, which are side products of pyrolysis. Note that the high flow spectrum reveals a decrease of the monomer peaks with respect to multimer peaks (2920–2980 cm), and the spectrum recorded on mass channel 28 reveals a decrease of most impurity related peaks (especially ethylene) with respect to ethyl peaks.

Image of FIG. 5.
FIG. 5.

Higher resolution scans of the black arrowed bands in Fig. 4 . The dashed red lines are a guide to the eye and coincide with the P(1), R(0), and R(1) line positions for the nuclear spin symmetry species (for which the top three spectra have been shifted to match up). The blue dashed lines coincide with the species Q(1) and R(1) lines in the second and fourth traces (for which only the bottom spectrum has been shifted). The symmetry species transitions are apparently heavily broadened in the third trace, as evidenced by the broad unresolved depletion signals underneath the partially resolved species lines. We note that the unique depletion profile in the bottom trace makes it difficult to firmly assign it to ethyl (the intensities are not expected on the basis of the group treatment). The weak peak at 3037.97 cm seems to correspond to ethyl and shows up in the band regardless of the precursor used (-propyl nitrite, di--amyl peroxide, or ethyl iodide); see Fig. S2.

Image of FIG. 6.
FIG. 6.

Higher resolution scans of the red arrowed bands in Fig. 4 . The most intense peak at 2884.74 cm in the third spectrum has been artificially suppressed. The peak marked with the asterisk seems to correspond to a larger cluster. For clarity, a best fit Lorentzian centred at 2990.88 cm, which corresponds to an impurity species, has been subtracted from the 3000 cm band.

Image of FIG. 7.
FIG. 7.

Comparison of the zero field and Stark spectra to simulations obtained using . The point group is assumed in the simulation and the constants employed are = 0.28 D (v = 0 and v = 1), = 3037.480 cm, ( + )/2 = 0.345 cm (v = 0 and v = 1), and = 0.0055 cm. The simulation is not sensitive to changes in the and components of the dipole moment.


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Table I.

Comparison of theoretical and experimental band characteristics for ethyl. Band positions in wavenumbers (cm).

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Table II.

Line characteristics for assigned transitions of ethyl (format is: line position (uncertainty) [line width]). Line positions and widths determined from Lorentzian fits, and uncertainty corresponds to difference between fitted line position and that as determined by-the-eye.

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Table III.

Spectroscopic constants for ethyl in helium nanodroplets (cm). Uncertainties are estimated from propagated errors in line positions.

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Table IV.

Relative integrated intensities (with set to one). He droplet values carry rather large estimated errors, at ±50%. Uncertainty in gas phase work is about ±12%.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: The ethyl radical in superfluid helium nanodroplets: Rovibrational spectroscopy and ab initio computations