Wafaa M. Fawzy

Transient diode laser absorption spectroscopy of the ν2 fundamental of trans‐HOCO and DOCO

We report the observation and assignment of the ν2 fundamental vibration in the HOCO and DOCO radicals. The radical was made by the photolysis of acetic acid or acetic acid‐d at 193 nm in a flow system. The observed spectra indicate that the excited vibrational level is perturbed in both HOCO and DOCO. In HOCO, the rotational levels in ν2=1 have an irregular dependence on the Ka quantum number, probably caused by anharmonic interactions with combinations of lower frequency in plane vibrations. An a‐type Coriolis interaction involving perturbation by a level containing one quantum of the out‐of‐plane torsional vibration cannot be rigorously excluded however. In DOCO, there are also N‐dependent perturbations in the excited state. Only parallel transitions were assigned in the ν2 fundamental of both isotopomers. The K=0 band origin for HOCO is at 1852.567 cm−1 which compares with 1843.7 cm−1 reported previously for the radical trapped in a low‐temperature argon matrix. This absorption spectrum will be useful...

Rotational, fine, and hyperfine structure in the high‐resolution electronic spectrum of ArOH and ArOD

A number of vibrational bands of the A 2Σ+↔X 2Π electronic spectrum of both ArOH and ArOD have been investigated by laser induced fluorescence with a high‐resolution, pulsed laser system yielding linewidths ≲250 MHz in the UV. This spectrum not only displays completely resolved rotational structure, but also fine and hyperfine structure. The hyperfine constants and precise interatomic distances derived from the rotational constants provide a very interesting picture of the electronic and geometric structure of the complex. The bonding is incipiently chemical in the A state with clear evidence for at least some electronic reorganization between Ar and the open‐shell OH radical in the complex. Conversely, the X state appears to be bound almost solely by physical van der Waals interactions characteristic of systems containing only closed‐shell species.

Rotational energy levels and line intensities for 2S+1Λ-2S+1Λ and 2S+1(Λ ± 1)-2S+1Λ transitions in a diatomic molecule van der Waals bonded to a closed shell partner

Abstract Hamiltonian matrix elements needed for calculating rotational energy levels are derived for a planar complex consisting of an open-shell diatomic molecule and a closed-shell partner. These matrix elements take account of spin-orbit interaction and a Renner-Teller-like splitting term, but not of the effects of large-amplitude internal rotation of the diatomic fragment within the complex. Rotational levels obtained by numerically diagonalizing this Hamiltonian matrix for given J are, as expected, strongly influenced by the degree of quantization of projections of the electron orbital, electron spin, and total angular momentum along the two natural axes in the problem, i.e., by the degree of quantization along the internuclear axis of the open-shell diatomic fragment, or along the inertial a axis of the near-symmetric-top planar complex. The degree of quantization of these varioss projections is in turn determined by the relative sizes of the spinorbit interaction, the Renner-Teller interaction, and products of the rotational constants and quantum numbers of the form BJ , CJ , and AK , as well as by the angle between the diatomic internuclear axis and the inertial a axis of the complex. Transition-moment matrix elements needed for calculating intensities in spin and orbitally allowed transitions in the open-shell diatomic fragment are also derived. Such transitions have the form 2 S +1 Λ- 2 S +1 Λ and 2 S +1 (Λ ± 1)- 2 S +1 Λ, where Λ = Σ, Π, Δ, Φ, etc. A brief discussion of how to use the Hamiltonian and transition-moment matrix elements in a computer program is given.

P‐type doubling in the infrared spectrum of NO–HF

The HF stretching band of the NO–HF open‐shell complex has been recorded using a molecular‐beam optothermal spectrometer. The spectrum exhibits P‐type doubling indicative of an unpaired electron spin coupled to the rotational angular momentum of a bent complex with substantially quenched electron orbital angular momentum. From B‘=0.111 320(17) cm−1, and an off‐axis angle for the NO of 30°, the zero‐point center‐of‐mass separation is estimated to be 3.4396(3) A. The HF frequency shift of 84 cm−1 indicates that the complex is hydrogen bonded, and the spectral intensities imply that the HF axis is aligned closely to the center‐of‐mass axis and the NO is off axis by 30±15°. The Renner–Teller‐like orbital quenching parameter is somewhat larger than the spin–orbit constant in the free NO molecule and increases substantially upon vibrational excitation. The transitions in this band exhibit vibrational predissociation broadening of 200±40 MHz (FWHM), similar to that observed for a number of closed‐shell hydrogen...

Infrared diode laser spectroscopy of the ν3 fundamental of the CD3 radical

The infrared absorption spectrum of the ν3 fundamental band of the CD3 radical has been detected by diode laser absorption spectroscopy. The CD3 radical was produced by excimer laser photolysis of CD3I at 248 nm or (CD3)2CO at 193 nm. Molecular parameters of the v3=1 vibrational state were determined from a least‐squares fit to 62 rotation–vibration transitions. In this fit, molecular parameters describing the ground state were constrained to those obtained from previous spectroscopic studies of the ν2 parallel IR band [J. M. Frye, T. J. Sears, and D. Leitner, J. Chem. Phys. 88, 5300 (1988)]. The molecular parameters determined in the present work are the band origin ν0=2381.088 60(84), B’=4.758 737(40), C’=2.373 297(34), (ζC)3=0.476 278(72), q3=0.003 76(59), D’N =0.000 187 9(5), DNK =−0.000 341 0(12), D’K =0.000 143 7(8), ηN =−0.000 005 5(36), η’K =0.000 060(35), and qN =0.000 063(17), all in cm−1 with one standard deviation in parentheses. The derived molecular parameters were compared with those for th...

Correlated ab initio study of the ground electronic state of the O2–HF complex

In this paper, we present the first correlated ab initio investigations on the ground electronic state of the O(2)-HF complex. Calculations were performed using the CCSD(T) method with the aug-cc-pVDZ and aug-cc-pVTZ basis sets. The results show that there are two equivalent minimum energy hydrogen-bonded structures of planar bent geometry, where the minima correspond to exchange of the oxygen atoms. For each minimum the length of the O-H hydrogen bond is 2.16 A. The best calculated value of D(e) of the equivalent minima is 271 cm(-1). The T-shaped geometry of the complex, with oxygen perpendicular to the axis connecting the center of masses of O(2) and the HF molecule, represents a barrier to tunneling between the equivalent minima. The best estimated value of that barrier height is 217 cm(-1). The linear O-O-HF geometry of the complex represents a saddle point. The calculated geometrical parameters of the minimum energy structure of the complex are in reasonable agreement with the previously reported spectroscopic results. However, results of the current calculations suggest that a full understanding of the fine structures of the observed infrared spectrum of the complex requires the development of an effective Hamiltonian that takes the effects of tunneling into account.

Observation and analysis of the infrared spectra of O2–HF near 3950 cm−1 and O2–DF near 2900 cm−1

Spectra were recorded in the H–F stretching fundamental region for O2–HF and in the D–F region for O2–DF, using a laser difference-frequency spectrometer coupled to a slit-nozzle expansion. By varying the ratio of oxygen to carrier gas, beam temperatures ranging from 5 to 16 K were obtained. One standard uncertainty for the relative frequency position of unblended lines is 0.0001 cm−1. Each spectrum was visually subdivided into a stronger (cold) spectrum and a weaker (hot) spectrum. Lines in the cold spectrum were fit to nearly experimental error, using a rotational Hamiltonian for open-shell complexes taken from the literature. For O2–DF, 21 rotational and spin–rotational parameters (10 each for the upper and lower state plus the band origin) were used to fit 86 transitions to a standard deviation of 0.0002 cm−1. For O2–HF, 23 rotational and spin–rotational parameters were used to fit 83 transitions to a standard deviation of 0.0003 cm−1. The slightly poorer quality of the fit for O2–HF than for O2–DF is...

Comment on “Theory of rotational energy levels of open-shell complexes containing the O2 molecule” [J. Chem. Phys. 107, 7651 (1997)]

An effective Hamiltonian was reported several years ago [Fawzy, J. Mol. Spectrosc. 160, 84 (1993)] on the effects of electron-spin on rotational energy levels of an open-shell complex containing a diatomic radical (in a 2S+1Λ electronic state, where Λ=0 for a Σ state, Λ=1 for a Π state, etc.; S⩾1/2) and a closed-shell partner. Recently, a paper was published [Qian, Low, Seccombe, and Howard, J. Chem. Phys. 107, 7651 (1997)] on rotational energy levels of an open-shell complex consisting of the O2 radical (in the 3Σg− state) and a closed-shell molecule. Even though the effective Hamiltonian of a complex containing oxygen can be easily obtained by simply setting L=Λ=0, S=1 in the 1993 model, the authors of the recent paper completely ignored these earlier results. Here, we present a comparison between the results of our least-squares fits of the reported infrared spectrum of the O2–N2O complex and those published by Qian et al. [Qian, Seccombe, and Howard, J. Chem. Phys. 107, 7658 (1997)]. The comparison sh...