Laser spectroscopy of FeO: Rotational analysis of some subbands of the orange system

Extensive laser excitation spectra and rotationally resolved laser-induced fluorescence spectra have been recorded for the “orange system” of gaseous FeO in the wavelength regions 5790–6140 and 5580–5640 Å. Detailed rotational analyses have been performed for about $\text{20}\text{Ω}\text{′}$ substates lying between 16 350 and 18 550 cm^?1. These are found to comprise a very severely perturbed ⁵Δ_i excited electronic state with a bond length of about 1.69 Å (which is responsible for the parallel polarization of the electronic transition from the ⁵Δ_i ground electronic state) and a large number of “extra” $\text{Ω}$ substates with B′ values ranging from 0.38 to 0.50 cm^?1, which almost certainly belong to high vibrational levels of lower-lying electronic states. Evidence about the natures of the “extra” states is confusing, however, with the ⁵⁴FeO⁵⁶FeO isotope shifts apparently being in conflict with the patterns of vibrationally resolved laser-induced fluorescence. Every single $\text{Ω}$ substate that has been analyzed shows rotational perturbations of varying severity. The density and magnitude of the rotational perturbations are quite exceptional for a diatomic molecule, and result in a new type of totally chaotic diatomic spectrum. There is a remarkable similarity to the visible spectrum of NO₂: in NO₂ the complications arise from the high density of perturbing ground state vibrational levels; in FeO there is a correspondingly high density of perturbing electronic states at lower energy. The great complexity of the FeO spectrum arises because the states are in an awkward intermediate spin-coupling case which still resembles Hund's case (a) but shows strong tendencies toward Hund's case (c) coupling.     

Radiative lifetimes and electronic quenching rate constants for single-photon-excited rotational levels of no (A2Σ+, ν′ = 0)

A narrow-band, frequency-doubled, tunable dye laser has been used to excite fluorescence from the A²Σ⁺, ν′ = 0 state of NO. Collision-free lifetimes were measured for 21 different K′ levels giving a mean radiative lifetime τ = 217 ± 4 ns. Electronic quenching rate constants of NO (A²Σ⁺, ν′ = 0) were measured for O₂, N₂, H₂O, CO₂ and Ar. No dependence of the quenching-rate constant on the initially excited rotational level was observed.     

Lifetime-measurements of alkali-molecules excited by different laserlines

Absorbing different lines of acw-argon ion laser or a He-Ne-laser the alkali molecules Na₂, K₂, Rb₂ and Cs₂ can be pumped into different rotational-vibrational levels of an excited electronic state. The lifetimes of these states have been measured using the phase shift technique at a modulation frequency of 18 Mc/sec. Because of collisions with the always present alkali atoms the excited molecules may undergo collision-induced transitions to other states and their lifetimes are dependent on the atomic pressure. This dependence was measured and the spontaneous lifetimes were obtained by extrapolation against zero pressure. Different collision processes are discussed which compete with the unperturbed fluorescence.     

Quantum ergodicity in small molecules: The continuum fluorescence of nitrogen dioxide

The failure of selection rules on K_a, v, and vibronic symmetry in the visible band systems of NO₂ are interpreted as resulting from the coupling of the excited electronic state with vibrational levels of the ground electronic state which are above the threshold for ergodic motion and therefore retain no quantization of those observables. This failure is shown to lead directly to the anomalous continuum fluorescence of NO₂, and is intimately related to the anomalous lengthening of the radiative lifetime of the excited state (the Douglas effect). It is predicted that most molecules which exhibit anomalous lifetime lengthening will also exhibit anomalous selection rules and, consequently, anomalous continuum emission.     

Intensity distributions of fluorescence from the 2B1 state of NO2 excited at 495.0, 474.0, 454.5, and 436.7 nm

Intensity distributions of the fluorescence spectra from the ²B₁ state of NO₂ excited at 495.0, 474.0, 454.5, and 436.7 nm were measured. The vibrational energy levels of the ²B₁ state and the fluorescence intensities from these levels were calculated by means of the theory of the Renner effect developed by Renner, Pople and Longuet-Higgins, and Dixon. Calculated band origins and fluorescence intensities agree with the present measurements, if the vibrational quantum number v′₂ = 15 is assigned to the upper state of the 454.5-nm Douglas-Huber band.     

Collisional transfer of rotational energy in the 2B1 electronic state of nitrogen dioxide

Collisional satellite lines have been observed in fluorescence from nitrogen dioxide excited by the 4545-Å line of the argon laser. The 13_0,13 level of the (0, 8, 0) vibrational state is populated by the laser and undergoes collisionally induced transitions to the 11_0,11, 15_0,15, and 17_0,17 states. These collisionally populated states are identified by their fluorescence to the well-studied (0, 0, 0) and (0, 1, 0) levels of the ground electronic state. These satellite lines are also observed in fluorescence to the (0, 2, 0) and (0, 3, 0) vibrational levels of the ground electronic state. The wavenumbers of those lines, together with those from unrelaxed fluorescence and previously published microwave transitions, allow vibrational and rotational constants for the higher vibrational states to be determined more accurately than was previously possible. Several much weaker forbidden transitions have also been observed, including ΔK_a = 0 through ?6 transitions in the (0, 8, 0)-(0, 0, 1) band.     

The Rotational Analysis of the Five new Vibronic Bands of No2 in the Range 5050-5200 Å

The five new vibrational bands in the range 5050-5200 Å of the laser induced fluorescence excitation spectra of NO₂ were measured and rotationally assigned at room temperature. Though the spectra were rather congested, we can determine the band origins, and rotational and spin-rotation constants for these bands. All rotational structures analyzed are of the parallel type. It was shown that the electronic excited state òB₂ were heavily perturbed by the high lying vibration levels of ground state [Xtilde] ² A ₁ and that the interactions between these two electronic states was the main rationale for the complexity of NO₂ visible spectra.     

Laser fluorescence spectroscopy of 2B1 (K > 0) vibronic states of NO2

The low-pressure fluorescence spectrum of NO₂ excited by the narrow (0.5 GHz) HeCd laser transition at 441.6 nm is found to be quite complex, indicating a rather high density of transitions to final states at this energy. Seven fluorescing states exhibiting perpendicular selection rules have been assigned to ²B₁ (K > 0) vibronic states of NO₂. From the presence of final states with v₃″ = 2, a considerable amount of ²A₂ electronic state character is attributed to at least some of these states.     

Infrared laser magnetic resonance (LMR) spectroscopy of NO2 (ν2) with a CO2 laser

The infrared laser magnetic resonance spectra for the ν₂ band of NO₂ were observed by using a CO₂ laser. High-K vibration-rotation transitions from ^rR₆(N) to ^rR₁₁(N) (v₂ = 1 ← 0) were observed. The analysis yielded some molecular parameters including two g factors for the excited vibrational state (v₂ = 1).     

Laser excited fluorescence spectrum of nitrogen dioxide

The fluorescence spectrum of NO₂ excited by a single mode argon-ion laser was studied at a spectral resolution of 0.1 cm^?1. In particular the Stokes and the anti-Stokes ν₂ band excited by the 5145 Å line of an argon-ion laser were examined in detail. Some of the vibrational and rotational parameters of the ground electronic state and the rotational constants of the B₂ vibronic state were obtained.     

Fluorescence and fluorescence excitation spectra of jet-cooled carbazole

The fluorescence excitation spectra of jet-cooled carbazole molecules at vibrational temperatures of 55 and 80 K and the fluorescence spectrum of these molecules excited by radiation at the frequency of a pure electronic transition are measured. As the vibrational temperature increases, the excitation spectra exhibit a series of lines of the same symmetry, which are caused by the interaction of the active vibration with a subensemble of optically inactive vibrations. The final symmetry of the totally and nontotally symmetric vibrations is determined from the shape of the rotational contours of the lines of vibronic transitions. The values of a decrease in the frequency of the nontotally symmetric vibrations in the first excited electronic state S ₁ due to their interaction with the electronic state S ₂ are calculated to be up to 100 cm^?1. The frequencies of the pure electronic transitions in the absorption and fluorescence spectra coincide with each other and are equal to 30809 cm^?1, the frequencies of vibrations in the ground state S ₀ exceeding the frequencies of the corresponding vibrations in the excited state S ₁. The degree of polarization of the integral fluorescence is determined for a series of vibronic transitions of the a ₁ and b ₂ final symmetry that are observed in the fluorescence excitation spectra, and the contribution of the intensity with the borrowed polarization θ_⊥ to the integral fluorescence is calculated. It is found that the intensity θ_⊥ is higher for the transitions of the b ₂ symmetry and can reach ≈50%.     

A build-up cavity for Fourier transform emission experiments

We have recorded electronic spectra of some diatomic species (I₂, K₂, and NaK), to illustrate the potential power of the combination of two high resolution techniques: intra-cavity laser induced fluorescence (ICLIF) and Fourier transform (FT) spectroscopy. Active and passive optical cavities have been used, working with visible continuous wave (cw) laser sources. The active cavity is a modified commercial ring dye laser, allowing for a sample up to 25 cm in length. Dispersed fluorescence spectra recorded on a Bomem Fourier transform spectrometer showed a signal enhancement of about 10 when a molecular source was placed within the resonator. The system was tested with a heatpipe source, producing alkali metal vapour at about 300 °C. These experiments illustrate enhanced cascade excitation mechanisms in K₂; the highest vibrational levels of the electronic ground state of K₂ can be observed with surprising ease. The increase in available power within the cavity has also led to the observation of fluorescence in NaK excited by a two-photon transition (Q (66) 6¹Σ⁺ ← X¹Σ⁺ transition). Spatial limitations have driven us to build a more versatile ring cavity able to accommodate larger sources. This broad-band (590-650 nm) build-up cavity is locked by a Hänsch-Couillaud servo-loop to an input laser of (instantaneous) bandwidth ∼1 MHz. Power enhancement factors of around 30 have been obtained with a 2.6% input coupler. The performance of the build-up cavity has been tested by recording FT spectra of intra-cavity laser induced fluorescence of iodine. The technique clearly has useful applications for weakly absorbing species, or for those whose electronic states are inaccessible to single-photon absorption techniques. This paper describes the arrangement we have used, highlighting some of the advantages and describing some of the particular difficulties we have encountered.     

Laser-induced fluorescence of the CH molecule in a low-pressure flame

The laser-induced fluorescence from [A²Δ(υ′ = 0)→X²Π(υ″ = 0)] band of the CH radical was studied in a low-pressure (20 torr) methane-oxygen flame (φ = 1.06). A time-resolved fluorescence technique was used to measure the relative CH concentration profile and the quenching of the A²Δ excited state through the flame. The pressure dependence of the quenching was also measured and used to determine an effective quenching cross section of 6 Ų in the CH₄-O₂ flame. Analysis of the fluorescence spectra scanned at different delays after the laser excitation, according to a pseudo-three-level model, yields a rotational energy transfer (RET) rate in the A²Δ(υ′ = 0) electronic state which is a factor of four faster than the electronic quenching rate of 1.57 × 10⁷ sec^-1 in the flame at 2000 K.     

Laser-Induced Fluorescence from CuCl2in a Free-Jet Expansion: High-Resolution Fourier Transform Spectra

High-resolution Fourier transform spectra of the laser-induced fluorescence of copper dichloride produced in a free-jet expansion are presented. The supersonic molecular beam produces rotational cooling and a nearly collision-free environment. Dispersed fluorescence spectra are found to be free of the continuum which restricted the data obtainable with a classical source (hot cell). The reduction in thermal emission allows the fluorescence to be recorded from the laser line (585 nm) out to 1.3 μm, revealing many new vibrational levels of CuCl₂in its ground electronic state,X²Π_g(3/2).     

Tunable laser study of the ν10 + ν11 band of cyclopropane

Diode laser measurements of the ν₁₀ + ν₁₁ (l_tot = ±2) perpendicular band of cyclopropane have led to the assignments of roughly 600 lines in the 1880–1920-cm^?1 region. Most of the spectra were recorded and stored in digital form using a rapid-scan mode of operating the laser. These spectra were calibrated, with the aid of a computer, by reference to the R lines of the ν₁ + ν₂ band of N₂O. The ground state constants we obtained are (in cm^?1) B = 0.670240 ± 2.4 × 10^?5, D_J = (1.090 ± 0.054) × 10^?6, D_JK = (?1.29 ± 0.19) × 10^?6, D_K = (0.2 ± 1.1) × 10^?6. The excited state levels are perturbed at large J values, presumably by Coriolis couplings between the active E′(l_tot = ±2) and the inactive A′(l_tot = 0) states. Effective values for the excited state constants were obtained by considering only the J < 15 levels. The A₁-A₂ splittings in the K′ = 1 excited states were observed to vary as q_effJ(J + 1), with q_eff = (2.17 ± 0.17) × 10^?4 cm^?1.     

Microwave optical double resonance and continuous wave dye laser excitation spectroscopy of NO2: Rotational assignment of the K = 0−4 subbands of the 593 nm band

A rotational assignment of approximately 80 lines with K_a′ = 0, 1, 2, 3, and 4 has been made of the 593 nm ${}^2\text{A}{}_1\text{→}{}^2\text{B}{}_2$ band of NO₂ using cw dye laser excitation and microwave optical double-resonance spectroscopy. Rotational constants for the ²B₂ state were obtained as A = 8.52 cm^?1, B = 0.458 cm^?1, and C = 0.388 cm^?1. Spin splittings for the K_a′ = 0 excited state levels fit a simple symmetric top formula and give $\text{(?}{}_{\mathrm{bb}}\text{+ ?}{}_{\mathrm{cc}}\text{)}\text{2}\text{= ?0.0483}\text{cm}{}^{\mathrm{?1}}$. Spin splittings for K_a′ = 1 (N′ even) are irregular and are shown to change sign between N′ = 6 and 8. Assuming that the large inertial defect of 4.66 amu Ų arises solely from A, a structure for the ²B₂ state is obtained which gives $\text{r}\text{(NO) = 1.35}\text{A}\text{?}$ and an ONO angle of 105°. Alternatively, weighting the three rotational constants equally gives $\text{r = 1.29}\text{A}\text{?}$ and θ = 118°.     

The rotational resonance Raman spectrum of nitrogen dioxide and the determination of electronic state symmetry from resonance Raman selection rules

The pure rotational Raman spectrum of nitrogen dioxide has been observed and shown to be consistent with existing determinations of molecular parameters. Upon observation at 600 Torr pressure and 0.4 cm⁻¹ resolution a well-defined rotational spectrum is obtained. This spectrum is overlaid with a number of fluorescence lines. The fluorescence lines are separated from the Raman spectrum by a comparison of Stokes and anti-Stokes branches of the rotational spectrum. Out of seven strong fluorescence lines seen with 5145 Å excitation, five probably are identifiable with vibration-rotation fluorescence progressions observed by Abe.The most striking feature of these observations is the potential use of the resonance Raman effect for the analysis of complicated electronic spectra. When this rotational spectrum is observed with excitation by 5309 Å or 5145 Å excitation, the Raman spectrum follows a-axis selection rules and the Q-branches are in the noise level or barely out of it. However, at 4880 Å the ΔK = 2Q-branches become a major feature of a spectrum, indicating that an appreciable part of the absorption at this wavelength is occurring through the operation of b- or c-axis selection rules. These findings are consistent with present notions of a ²B₂ excited state dominating absorption at longer wavelengths, while at shorter wavelengths a ²B₁ excited state becomes important. Given a tunable laser, one could map the relative importance of these two possible selection rules for NO₂ without any theoretical analysis more sophisticated than that presented in this paper.A simplified statement of the selection rules for resonance rotational Raman spectra of asymmetric tops has been developed in the course of this investigation. No attempt has been made to refine the rotational parameters of NO₂ since all of the lines seen areunresolved multiplets. Our data should be regarded as a search spectrum preliminary to investigation on a high resolution instrument.     

Anomalous magnetic depolarization of fluorescence from the NO22B2 state

Hanle-effect studies were made of 13 identified lines belonging to the 5933-, 6117-, 6125-, and 6126-Å bands of the NO₂²B₂ state. The fluorescence was excited using a tunable cw dye laser operated single mode, using NO₂ pressures as low as 0.2 mTorr. From the variation of the fluorescence intensity with magnetic field the product gτ_C is deduced, where g and τ_C are the excited state g value and coherence lifetime, respectively. Using previously determined radiative lifetimes, g is obtained and compared to the values of g expected from Hund's case (b) coupling. The observed g values do not display the expected N′ dependence and are about an order of magnitude smaller than predicted. An alternative explanation is that the g values are behaved, but the coherence lifetime is shorter than the radiative lifetime, caused by radiationless dephasing processes which may be collisional or intrinsic.     

The valence states of NO+

Spectroscopic constants for the eight lowest electronic states of the NO⁺ ion are tabulated. These constants result from reanalysis of previously reported optical and photoelectron spectra and interpolation from corresponding states of the isoelectronic molecules CO and N₂. Similar spin-orbit perturbations of the A¹Π states of NO⁺ and CO are compared. An interpretation is given of previously reported emission from a beam of long-lived states of NO⁺. The intensities of ionizing transitions from NO X²Π (v = 0) observed in photoelectron spectra are compared with calculated Franck-Condon factors.