Robert Kuhn

Emission spectra of supersonically cooled chloroacetylene cations

Abstract The Ā2ΠΩ → X2ΠΩ, Ω = 3 2 , 1 2 emission spectra of rotationally cooled chloro- and deuterochloro-acetylene cations have been obtained by electron-impact ionisation of a seeded helium supersonic free jet. The resultant spectral improvements lead to the identification of the spin-orbit components and isotope splittings and to a vibrational assignment of the prominent bands. The vibrational frequencies could be determined to within ±1 cm−1 for many of the fundamentals for the cations in the X2ΠΩ and Ā2ΠΩ electronic states.

Emission spectra of supersonically cooled dimethyldiacetylene cations

Abstract The Ā2E → X2E (Σ= + 1 2 , - 1 2 ) electronic transitions of rotationally/vibrationally cooled CH3CCCCH- cation, as well as the d1-/d3-/d4-substituted species, were studied by emission spectroscopy. Ion emission was obtained by electron impact on the neutral species seeded in a helium supersonic free jet. Vibrational frequencies in both electronic states are inferred to within ±1 cm-1. Spin-orbit splittings are observed and interpreted on the basis of non-linear vibronic couplings. Rotational subbands are observed, yielding rotational and Coriolis parameters as well as rotational temperatures.

A reinvestigation of the CBr+ chemiluminescence in a helium afterglow

Abstract The emission spectrum produced from the flowing He afterglow reaction of CBr4 has been re-examined. The a3Πo▾ → X1Σ+ emission subsystem of CBr+ is identified in addition to the a 3Π1 → X1Σ+ subsystem reported previously. The vibrational constants of the a3Πo▾ substrate are estimated to be (in cm−1)Te = 22449 ± 2, ωe = 906 ± 2, ωexe = 7.8 ± 0.3, and the a3Π, state spin-orbit splitting constant of CBr+ is 369 ± 8 cm−1. A new ultraviolet emission system (2650–2900A) is tentatively assigned to the A1Π → X1Σ+ transition of CBr+.

Spectroscopic characterization of organic cations: emission and laser-excitation spectra of rotationally cooled CH3CCX+, X Cl, Br

Abstract The gas phase emission and laser-excitation spectra of the A 2 E ↔ X 2 E transition of rotationally and vibrationally cooled 1-chloro- and 1-bromopropyne cations, and their fully deuterated analogues, have been obtained. The emission was excited by electron impact on a seeded helium supersonic free jet and the fluorescence by laser-excitation of cations formed by Penning ionization and collisional relaxation of liquid-nitrogen temperature. Comparison of the two sets of data locates the band origins and enables the prominent spectral features to be vibrationally assigned. The vibrational frequencies of many of the fundamentals could be inferred for these cations in their X 2 E and A 2 E states.

Ā2A1 → X̄2E emission of 1-phosphapropyne cation, CH3CP+

The Ā2A1 → X2E emission spectrum of CH3CP+ in the gas phase has been observed in the 530–590 nm region. The cations were produced by electron impact on either an effusive beam or seeded helium supersonic free jet or CH3CP. Two pairs of spin-orbit separated bands are identified: O00, OOO and 2O1, 2O1. The derived constants are (in cm−1): T0=18656(1), a″O=−85(2) and ν″2=1503(2).

Photoelectron-photon coincidence studies of the A

Abstract Fluorescence quantum yields and lifetimes of the above given cations in selected levels within their lowest excited electronic states have been measured by a photoelectron—photon coincidence technique. These data, obtained under collision-free conditions, lead to the radiative and non-radiative rate constants as a function of the internal energy. The symmetry of the A

̃and B

state is 2 A 1 (X = CH 3 , CD 3 ), 2 Σ + (X = Cl), but 2 Π (X = Br, I) and the corresponding k r values for these two groups, 1–2 × 10 6 s −1 and 2 × 10 7 s −1 respectively, reflect the different nature of the transitions. Other essential features of the results are discussed.

̃excited electronic states of X-CC-CN+, X = CH3, CD3, Cl, Br, I

Abstract The vibrational frequencies of bromocyanoacetylene cation in its ground and first excited electronic states have been obtained by recording and analysing the emission and laser excitation spectra of the A 2 π- X 2 π transition. In the emission experiments the ions are produced rotationally cooled to ca. 10 K by means of a supersonic free jet and in the laser excitation measurements to ca. 100 K by Penning ionisation followed by collisional relaxation. The resulting narrowing of the vibronic bands in the spectra is such that the vibrational frequencies of most of the fundamentals could be inferred to within ± 2 cm −1 .