Infrared spectroscopy of jet-cooled transient molecules

R. Curl, K.K. Murray, M. Petri, M. Richnow, F. Tittel, Infrared spectroscopy of jet-cooled transient molecules, Chem. Phys. Lett., 161 (1989) 98-102; doi: 10.1016/0009-2614(89)85038-9

Abstract

Pulsed valve jet-cooled radical setup
Pulsed valve jet-cooled radical setup: excimer laser fires toward (or across) the pulsed valve jet expansion to create radicals that are cooled in the expansion.

Free radicals are produced by excimer laser flash photolysis of a suitable precursor inside the nozzle of a pulsed slit jet and cooled by supersonic expansion from the slit. The infrared spectra of the radicals are then probed by a single-frequency color-center laser. This technique has been tested on the NH2 radical.

Photoelectron spectroscopy of halocarbene anions

K.K. Murray, D.G. Leopold, T.M. Miller, W.C. Lineberger, Photoelectron spectroscopy of the halocarbene anions HCF, HCCl, HCBr, HCI, CF2, and CCl2, J. Chem Phys, 89 (1988) 5442-5453; doi: 10.1063/1.455596

Abstract

The 488 nm photoelectron spectra are reported for the HCX(X̃1A’)+e−←HCX−(X̃2A‘) and HCX(ã3A‘)+e−←HCX−(X̃2A‘) transitions in HCF−, DCF−, HCCl−, HCBr−, and HCI− and for the CX2(X̃1A1)+e−←CX−2(X̃2B1) transitions in CF−2 and CCl−2 . Adiabatic electron affinities are found to be 0.557±0.005 eV (HCF), 0.552±0.005 eV (DCF), 1.213±0.005 eV (HCCl), 1.556±0.008 eV (HCBr), 1.683±0.012 eV (HCI), 0.179±0.005 eV (CF2), and 1.603 ± 0.008 eV (CCl2). Bounds for the triplet excitation energies are determined for all the halocarbenes for which photoelectron spectra were observed, with the exception of CCl2. For the HCX halocarbenes, upper bounds for the triplet excitation energies are 14.7±0.2 kcal/mol (HCF, DCF), 11.4±0.3 kcal/mol (HCCl), and 9±2 kcal/mol (HCBr). A more detailed analysis of HCF suggests the actual triplet excitation energy to be 11.4±0.3 kcal/mol, 14.7±0.2 kcal/mol, or 8.1±0.4 kcal/mol, with the first value the most likely. Since the multiplicity of the ground state of HCl is not known, we report the energy of its first excited state to be less than 9±2 kcal/mol. The absence of an observed triplet state in the CF−2 photoelectron spectrum allows us to assign a lower bound on the triplet excitation energy of CF2 of 50 ± 2 kcal/mol.

Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of CH2−and CD2−

D.G. Leopold, K.K. Murray, A.E.S. Miller, W.C. Lineberger, Methylene: A study of the X̃ 3B1 and ã 1A1 states by photoelectron spectroscopy of CH2and CD2J. Chem Phys. 83 (1985) 4849. doi:10.1063/1.449746.

Abstract

Apparatus from Leopold, Murray, Miller and Lineberger,  J. Chem. Phys. 83, 4849 (1985).
Apparatus from Leopold, Murray, Miller and Lineberger, J. Chem. Phys. 83, 4849 (1985).

Photoelectron spectra are reported for the CH2(X̃ 3B1)+e←CH2 (X̃ 2B1) and CH2(ã 1A1)+e←CH2(X̃ 2B1) transitions of the methylene and perdeuterated methylene anions, using a new flowing afterglow photoelectron spectrometer with improved energy resolution (11 meV). Rotational relaxation of the ions to ∼300 K and partial vibrational relaxation to <1000 K in the flowing afterglow negative ion source reveal richly structured photoelectron spectra. Detailed rotational band contour analyses yield an electron affinity of 0.652±0.006 eV and a singlet–triplet splitting of 9.00±0.09 kcal/mol for CH2. (See also the following paper by Bunker and Sears.) For CD2, results give an electron affinity of 0.645±0.006 eV and a singlet–triplet splitting of 8.98±0.09 kcal/mol. Deuterium shifts suggest a zero point vibrational contribution of 0.27±0.40 kcal/mol to the observed singlet–triplet splitting, implying a Te value of 8.7±0.5 kcal/mol. Vibrational and partially resolved rotational structure is observed up to ∼9000 cm−1 above the zero point vibrational level of the 3B1 states, revealing a previously unexplored region of the quasilinear potential surface of triplet methylene. Approximately 20 new vibration‐rotation energy levels for CH2 and CD2 are measured to a precision of ∼30 cm−1 in the v2=2–7 region (bent molecule numbering). Bending vibrational frequencies in the methylene anions are determined to be 1230±30 cm−1 for CH and 940±30 cm−1 for CD2, and the ion equilibrium geometries are bracketed. The measured electron affinity also provides values for the bond strength and heat of formation of CH2, and the gas phase acidity of CH3. A detailed description of the new flowing afterglow photoelectron spectrometer is given.

Laser photoelectron spectroscopy of vibrationally relaxed CH2−: A reinvestigation of the singlet–triplet splitting in methylene

D.G. Leopold, K.K. Murray, W.C. Lineberger, Laser photoelectron spectroscopy of vibrationally relaxed CH2: A reinvestigation of the singlet–triplet splitting in methylene, J. Chem Phys. 81 (1984) 1048–1050. doi:10.1063/1.447741.

CH2- photoelectron spectra following 2.54 eV (488 nm) excitation.
CH2 photoelectron spectra following 2.54 eV (488 nm) excitation. (a) Previously reported spectrum (Zittel 1976, Engelking 1981) obtained with a gas discharge ion source. (b) Spectrum of vibrationally and rotationally cooled CH2 prepared in a flowing afterglow ion source. The instrumental resolution is 60 meV in (a) and 10 meV in (b). The absence of peaks A, B, and C in (b) establishes their hot band nature and gives II singlet-triplet splitting of 9 kcal/mol.

In view of the many questions concerning the photoelectron spectrum of CH2 we have reinvestigated this system using a new experimental apparatus which incorporates a flowing afterglow ion source, providing vibrational and rotational cooling of the sample ions. Results presented here enable several previously observed spectral features to be positively identified as vibrational hot bands, leading to a revised determination of the singlet-triplet splitting in methylene.

Wein filter magnet power supply
Negative ion photoelectron spectrometer in 2009; Wein filter magnet power supply
Negative ion photoelectron spectrometer in 2009
Negative ion photoelectron spectrometer in 2009