Wavelength Resolved Emission Spectra Research Articles

Vibrational predissociation in the bending levels of the à state of C3Ar.

Vibrational predissociation (VP) has been observed in 16 bands of the C3Ar van der Waals complex near the 0 v2 0 - 000 (v2 = 2-, 4-, 2+) and 0 2- 2 - 100 bands of the Ã1Π-X̃1Σ+ g system of C3. New higher resolution wavelength-resolved emission (WRE) spectra covering a wider spectral range have been recorded for many of these C3Ar bands, which show that most of the features observed in fluorescence must be reassigned as emission from the C3 fragment. Two types of VP processes have been recognized. The first type gives rise to vibrationally hot C3 fragments, mostly following |Δv| = 1, |ΔP| = 1 propensity rules, where P is the vibronic angular momentum of C3. The second type gives vibrationally cooled fragments. The VP processes can change abruptly from one type to the other with comparatively small differences in vibrational energy. Although the initial states are associated with both orbital components of the C3, Ã1Πu state, most of the VP fragments belong to the lower orbital component. A dipole-induced dipole model has been used to interpret the observed ΔP- propensities. Ab initio calculations of the binding energies of the ground and excited electronic states of C3Ar have been carried out; the calculated values are consistent with estimates of ≤144 cm-1 and 164 cm-1, respectively, given by the WRE spectra.

Combined Spatially Resolved Optical Emission Imaging and Modeling Studies of Microwave-Activated H2/Ar and H2/Kr Plasmas Operating at Powers and Pressures Relevant for Diamond Chemical Vapor Deposition.

Microwave (MW) activated H2/Ar (and H2/Kr) plasmas operating under powers and pressures relevant to diamond chemical vapor deposition have been investigated experimentally and by 2-D modeling. The experiments return spatially and wavelength resolved optical emission spectra of electronically excited H2 molecules and H and Ar(/Kr) atoms for a range of H2/noble gas mixing ratios. The self-consistent 2-D( r, z) modeling of different H2/Ar gas mixtures includes calculations of the MW electromagnetic fields, the plasma chemistry and electron kinetics, heat and species transfer and gas-surface interactions. Comparison with the trends revealed by the spatially resolved optical emission measurements and their variations with changes in process conditions help guide identification and refinement of the dominant plasma (and plasma emission) generation mechanisms and the more important Ar-H, Ar-H2, and H-H2 coupling reactions. Noble gas addition is shown to encourage radial expansion of the plasma, and thus to improve the uniformity of the H atom concentration and the gas temperature just above the substrate. Noble gas addition in the current experiments is also found to enhance (unwanted) sputtering of the copper base plate of the reactor; the experimentally observed increase in gas phase Cu* emission is shown to correlate with the near substrate ArH+ (and KrH+) ion concentrations returned by the modeling, rather than with the relatively more abundant H3+ (and H3O+) ions.

Spatially Resolved Optical Emission and Modeling Studies of Microwave-Activated Hydrogen Plasmas Operating under Conditions Relevant for Diamond Chemical Vapor Deposition.

A microwave (MW) activated hydrogen plasma operating under conditions relevant to contemporary diamond chemical vapor deposition reactors has been investigated using a combination of experiment and self-consistent 2-D modeling. The experimental study returns spatially and wavelength resolved optical emission spectra of the d → a (Fulcher), G → B, and e → a emissions of molecular hydrogen and of the Balmer-α emission of atomic hydrogen as functions of pressure, applied MW power, and substrate diameter. The modeling contains specific blocks devoted to calculating (i) the MW electromagnetic fields (using Maxwell's equations) self-consistently with (ii) the plasma chemistry and electron kinetics, (iii) heat and species transfer, and (iv) gas-surface interactions. Comparing the experimental and model outputs allows characterization of the dominant plasma (and plasma emission) generation mechanisms, identifies important coupling reactions between hydrogen atoms and molecules (e.g., the quenching of H( n > 2) atoms and electronically excited H2 molecules (H2*) by the alternate ground-state species and H3+ ion formation by the associative ionization reaction of H( n = 2) atoms with H2), and illustrates how spatially resolved H2* (and Hα) emission measurements offer a detailed and sensitive probe of the hyperthermal component of the electron energy distribution function.

Electronic bands of scandium monocarbide, ScC, in the region 14 140-16 000 cm-1.

Scandium monocarbide molecules, ScC, have been prepared by the reaction of 532 nm laser-ablated Sc metal with acetylene or methane under supersonic jet-cooled conditions. Electronic spectra of Sc12C and Sc13C have been recorded in the region 14 140-16 000 cm-1 using laser-induced fluorescence, and about 40 bands of each isotopomer have been analyzed rotationally. Wavelength resolved emission spectra have been obtained for many of them. The results show that Sc12C has a 2Πi ground state, with a bond length of 1.952 Å. Its vibrational frequency and spin-orbit coupling constant are 648 cm-1 and -39.47 cm-1, respectively (631 cm-1 and -39.32 cm-1 in Sc13C). Lying 155.58 cm-1 above the X2Π3/2 level (154.72 cm-1 in Sc13C) is a 4Π5/2 level, the lowest spin-orbit component of a 4Πi state. The excited states at higher energy are very complicated. Bands from both the doublet and quartet spin manifolds are present, and there are strong doublet-quartet interactions which induce many nominally-forbidden bands violating the selection rule ΔS = 0. Eight excited electronic states have been recognized, including four 4Δ states. These 4Δ states represent four of the five 4Δ states from the electron configurations (C 2pσ)2 (C 2pπ)2 (Sc 3dδ)1 and (C 2pσ)1 (C 2pπ)2 (Sc 4sσ)1 (Sc 3dδ)1.

Hyperfine rather than spin splittings dominate the fine structure of the B (4)Σ(-)-X (4)Σ(-) bands of AlC.

Laser-induced fluorescence and wavelength resolved emission spectra of the B (4)Σ(-)-X (4)Σ(-) band system of the gas phase cold aluminum carbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high pressure argon. High resolution spectra show that each rotational line of the 0-0 and 1-1 bands of AlC is split into at least three components, with very similar splittings and intensities in both the P- and R-branches. The observed structure was reproduced by assuming bβS magnetic hyperfine coupling in the excited state, due to a substantial Fermi contact interaction of the unpaired electron in the aluminum 3s orbital. Rotational analysis has yielded ground and excited state equilibrium bond lengths in good agreement with the literature and our own ab initio values. Small discrepancies in the calculated intensities of the hyperfine lines suggest that the upper state spin-spin constant λ' is of the order of ≈ 0.025-0.030 cm(-1).

Toward an improved understanding of the AsH2 free radical: Laser spectroscopy, ab initio calculations, and normal coordinate analysis

Spectra of the Ã(2)A(1)-X̃(2)B(1) transition of the jet-cooled AsD(2) and AsHD isotopologues of the arsino radical have been studied by laser induced fluorescence and wavelength resolved emission techniques. A high-resolution spectrum of the AsD(2) 0(0)(0) band has been recorded, and an improved r(0) structure [r(0)(') = 1.487(4) Å, θ(0)(') = 123.0(2)°] for the à state has been determined from the rotational constants. To aid in the analysis of the vibrational levels, an ab initio potential energy surface of the X̃(2)B(1) state has been constructed and the rovibronic energy levels of states on that potential have been determined using a variational method. The vibrational levels observed in wavelength resolved emission spectra have been fitted to a local mode Hamiltonian with most anharmonic parameters fixed at ab initio values, and the resulting harmonic frequencies have been used to perform a normal coordinate analysis which yielded an improved set of quadratic force constants and an estimate of the equilibrium ground state structure.

A laser-induced fluorescence study of the jet-cooled nitrous oxide cation (N2O+)

Laser-induced fluorescence and wavelength resolved emission spectra of the à (2)Σ(+) - X̃ (2)Π(i) electronic transition of the jet-cooled nitrous oxide cation have been recorded. The ions were produced in a pulsed electric discharge at the exit of a supersonic expansion using a precursor mixture of N(2)O in high pressure argon. Both spin-orbit components of the 0(0) (0) band were studied at high resolution and rotationally analyzed to provide precise molecular constants for the combining states. Emission spectra were obtained by laser excitation of the 0(0) (0), 2(0) (1), 3(0) (1), and 2(0) (2) absorption bands, providing extensive data on the ground state bending, stretching, and combination vibrational levels. These data were fitted to a Renner-Teller model including spin-orbit, anharmonic, and Fermi resonance terms. The observed energy levels and fitted parameters were found to be comparable to those in the literature predicted from an ab initio potential energy surface.

Pulsed discharge jet electronic spectroscopy of the aluminum dicarbide (AlC2) free radical

Laser-induced fluorescence and wavelength resolved emission spectra of the C ̃(2)B(2)-X̃ (2)A(1) band system of the gas phase aluminum dicarbide free radical have been obtained using the pulsed discharge jet technique. The radical was produced by electron bombardment of a precursor mixture of trimethylaluminum in high-pressure argon. The three vibrational frequencies of T-shaped AlC(2) have been determined in both the combining states along with several of the anharmonicity constants. The 0(0)(0) band has been recorded with high resolution and rotationally analyzed. The spectrum is complicated by partially resolved spin-rotation and aluminum hyperfine splittings. Where necessary, we have fixed the spin-rotation constants used in the rotational analysis at the values predicted by density functional theory. The derived molecular structures are: r(0)('')(C-C) = 1.271(2) Å, r(0)('')(Al-C) = 1.926(1) Å, θ(")(C-Al-C) = 38.5(2)°, r(0)(')(C-C) = 1.323(2) Å, r(0)(')(Al-C) = 1.934(1) Å, and θ(')(C-Al-C) = 40.0(2)°. Unlike SiC(2), aluminum dicarbide shows no spectroscopic evidence of facile isomerization to the linear structure in the ground electronic state.

The C3-bending vibrational levels of the C3–Kr and C3–Xe van der Waals complexes studied by their Ã−X̃ electronic transitions and by ab initio calculations

Fluorescence excitation spectra and wavelength-resolved emission spectra of the C(3)-Kr and C(3)-Xe van der Waals (vdW) complexes have been recorded near the 2(2-)(0), 2(2+)(0), 2(4-)(0), and 1(1)(0) bands of the Ã(1)Π(u)-X̃(1)Σ(g)(+) system of the C(3) molecule. In the excitation spectra, the spectral features of the two complexes are red-shifted relative to those of free C(3) by 21.9-38.2 and 34.3-36.1 cm(-1), respectively. The emission spectra from the à state of the Kr complex consist of progressions in the two C(3)-bending vibrations (ν(2), ν(4)), the vdW stretching (ν(3)), and bending vibrations (ν(6)), suggesting that the equilibrium geometry in the X̃ state is nonlinear. As in the Ar complex [Zhang et al., J. Chem. Phys. 120, 3189 (2004)], the C(3)-bending vibrational levels of the Kr complex shift progressively to lower energy with respect to those of free C(3) as the bending quantum number increases. Their vibrational structures could be modeled as perturbed harmonic oscillators, with the dipole-induced dipole terms of the Ar and Kr complexes scaled roughly by the polarizabilities of the Ar and Kr atoms. Emission spectra of the Xe complex, excited near the Ã, 2(2-) level of free C(3), consist only of progressions in even quanta of the C(3)-bending and vdW modes, implying that the geometry in the higher vibrational levels (υ(bend) ≥ 4, E(vib) ≥ 328 cm(-1)) of the X̃ state is (vibrationally averaged) linear. In this structure the Xe atom bonds to one of the terminal carbons nearly along the inertial a-axis of bent C(3). Our ab initio calculations of the Xe complex at the level of CCSD(T)∕aug-cc-pVTZ (C) and aug-cc-pVTZ-PP (Xe) predict that its equilibrium geometry is T-shaped (as in the Ar and Kr complexes), and also support the assignment of a stable linear isomer when the amplitude of the C(3) bending vibration is large (υ(4) ≥ 4).

The electronic spectrum of the fluoroborane free radical. II. Analysis of laser-induced fluorescence and single vibronic level emission spectra

Subsequent to our spectroscopic detection of the HBX (X=F, Cl, Br) free radicals (S.-G. He, F. X. Sunahori, and D. J. Clouthier, J. Am. Chem. Soc. 127, 10814 (2005)), the electronic spectrum of the A (2)A(")Pi-X (2)A(') system of the fluoroborane (HBF) radical in the 600-745 nm region has been studied in detail using the pulsed discharge jet technique. The band system involves a linear-bent transition between the two Renner-Teller components of what would be a (2)Pi state at linearity. Using the results of our theoretical study of the ground and excited state vibrational energy levels and (11)B-(10)B isotope shifts (see the companion paper), the vibrational quantum numbers of the bands in the laser-induced fluorescence (LIF) spectra have been assigned. Rotational and vibrational analyses of the LIF and wavelength resolved emission spectra have been carried out, from which the linear excited state and the bent ground state equilibrium configurations have been confirmed. The ground state molecular geometry of HBF has been determined as r(0)(BH)=1.214(2) A, r(0)(BF)=1.303 4(5) A, and theta=120.7(1) degrees. Based on high-level ab initio calculations and symmetry considerations, predissociation of the excited state into H((2)S)+BF((1)Sigma(+)) on the ground state potential energy surface is identified as the cause of the breaking off of fluorescence in the LIF spectra.

Theoretical and experimental studies of collision-induced electronic energy transfer from v=–3 of the E(g+) ion-pair state of Br2: Collisions with He and Ar

Collisions of Br(2), prepared in the E(0(g)+) ion-pair (IP) electronic state, with He or Ar result in electronic energy transfer to the D, D', and beta IP states. These events have been examined in experimental and theoretical investigations. Experimentally, analysis of the wavelength resolved emission spectra reveals the distribution of population in the vibrational levels of the final electronic states and the relative efficiencies of He and Ar collisions in promoting a specific electronic energy transfer channel. Theoretically, semiempirical rare gas-Br(2) potential energy surfaces and diabatic couplings are used in quantum scattering calculations of the state-to-state rate constants for electronic energy transfer and distributions of population in the final electronic state vibrational levels. Agreement between theory and experiment is excellent. Comparison of the results with those obtained for similar processes in the IP excited I(2) molecule points to the general importance of Franck-Condon effects in determining vibrational populations, although this effect is more important for He collisions than for Ar collisions.

Spectroscopic Identification of C2P and C2As, Two New Main Group Dicarbides

The first spectroscopic evidence for the existence of the CCP and CCAs free radicals has been obtained from laser-induced fluorescence and wavelength-resolved emission spectra acquired in supersonic discharge jet experiments. The observed vibrational frequencies, 13C isotope effects, spin−orbit splittings, and rotational band contours have been used to positively identify the radicals. The measured ground state vibrational frequencies are in accord with our density functional molecular orbital theory predictions. The CCP species, which has a large calculated dipole moment, is a possible interstellar molecule that may be detectable by radioastronomy.

A Family of New Boron-Containing Free Radicals

The first spectroscopic evidence for the existence of the HBX and DBX (X = F, Cl, Br) free radicals has been obtained from laser-induced fluorescence and wavelength-resolved emission spectra. The vibrational frequencies measured in emission are in excellent agreement with our ab initio (CCSD(T)/aug-cc-pVTZ) predictions. The patterns of resolved rotational sub-band structure and deuterium isotope effects have been used to positively identify the radicals. The experimental data and ab initio predictions will be useful in searches for the matrix infrared and microwave spectra of these new radicals.

Wavelength resolved laser-induced fluorescence emission of [formula omitted] trapped in an ion cyclotron resonance cell

We have measured the wavelength resolved fluorescence emission laser-induced fluorescence (LIF) spectrum of C 6 F 3 H 3 + confined in an open cylindrical ion cyclotron resonance Penning trap. Ion cyclotron resonant azimuthal quadrupolar excitation successfully mass-selects and traps the ions for 10 s for repeated laser interrogation, and establishes that the observed fluorescence is from C 6 F 3 H 3 + ions. Electron-induced fluorescence (EIF) emission spectra of the same species were acquired for comparison. Vibrational structure was resolved for both LIF and EIF, and our assignments are consistent with prior literature values in the absence of ion trapping and high magnetic field (3 T). This work represents the first determination of wavelength resolved LIF emission spectra of organic ions in a Penning trap.

The electronic spectroscopy and molecular structure of the HPCl free radical: A potential III–V semiconductor growth intermediate

The à 2A′–X̃ 2A″ electronic spectra of jet-cooled HPCl and DPCl have been obtained for the first time using the pulsed electric discharge technique with a precursor mixture of PCl3 and H2 or D2. From a combination of laser-induced fluorescence and wavelength resolved emission spectra, all of the vibrational frequencies in the ground and excited states of both isotopomers have been measured and vibrational force fields have been determined. Rotational analyses of the 000 bands of both isotopomers showed small doublet splittings characteristic of an asymmetric top molecule with a single unpaired electron. From the rotational constants and the force fields, estimated equilibrium structures were derived with r″(PH)=1.4158(23) Å, r″(PCl)=2.0388(23) Å, θ″=95.02(27)°, and r′(PH)=1.4067(20) Å, r′(PCl)=2.0050(2) Å, and θ′=115.53(12)°. The experimental data firmly establish that the observed spectra and those previously obtained by chemiluminescence techniques [Bramwell et al., Chem. Phys. Lett. 331, 483 (2000)] are due to the HPCl free radical.

Exploring the Bermuda triangle of homonuclear diatomic spectroscopy: The electronic spectrum and structure of Ge2

The optical spectrum of jet-cooled Ge2 has been observed for the first time. Laser-induced fluorescence (LIF) and wavelength resolved emission spectra were recorded using the pulsed discharge technique with a tetramethylgermane precursor. Analysis of the spectra yielded the vibrational constants ωe″=287.9(47), ωexe″=0.81(55), ωeye″=0.0037(18), ωe′=189.0(15), ωexe′=6.41(30), and Te′=20 610.8(16) cm−1. High-resolution rotationally resolved spectra of several bands of Ge274 show two strong P and R branches and two very weak Q branches. We have assigned the band system as a Hund’s case (c) Ω′=1−Ω″=1 transition from the ground Σg−3 state to a Σu−3 excited state. The bond lengths derived from the rotational constants are r0″=2.3680(1) Å and re′=2.5244(18) Å, an ∼0.16 Å increase on electronic excitation. Arguments are presented for assigning the transition to a σg2πu2→σg2πuπg electron promotion, although the observed increase in the bond length is much less than predicted by previous ab initio calculations. The absence of the 0u+–0g+ component in the spectra has been attributed to an excited state predissociation.

Chemiluminescent production of CF 2 (Ã [formula omitted]) following the reaction O( [formula omitted])+C 2F 4

Emission from CF 2 (Ã 1 B 1 ) was observed following the photolysis of N 2O at 210 nm in the presence of C 2F 4. Wavelength-resolved emission spectra confirm CF 2 as the spectral carrier. The emission decay time and intensity varies with the partial pressure of C 2F 4; the emission is shown to result from the reaction between O( 1 D ) and C 2F 4. Analysis of the fluorescence decay rate as a function of C 2F 4 concentration yields a rate coefficient of (2.4±0.3)×10 13 cm 3/mol-s for the O( 1 D )+C 2F 4 reaction at 296 K.

Laser-induced excitation and dispersed fluorescence spectra of the ethoxy radical

Extensive laser excitation and dispersed fluorescence spectra of the ethoxy radical have been recorded in a supersonic jet expansion. Neon transitions have been used to calibrate both the wavelength of the excitation dye laser and the optical multichannel analyser system used to record the wavelength-resolved emission spectra. Both the excitation and dispersed spectra are characterized by prominent progressions involving the C - O stretch vibrational mode. Seven vibrational frequencies for the excited electronic state and eight for the ground state have been assigned. To the best of our knowledge, 10 of these 15 assigned frequencies are reported here for the first time. Vibrational and anharmonic constants for the C - O stretch mode have been determined via least-squares fits for the X state and the B state .

Laser-induced fluorescence spectroscopy of the jet-cooled methylthio radical

Laser-induced fluorescence excitation and wavelength-resolved emission spectra have been recorded for the methylthio (CH3S) radical in a supersonic jet environment. Twenty one vibronic bands have been obtained in excitation in the 360-387 nm region with about 0.2 cm-1 resolution and assigned to the A 2A1 - X 2E3/2 and A 2A1 - X 2E1/2 electronic transitions. Fluorescence from five vibronic bands (000, 301, 302, 312 and 201310) has been individually dispersed by a 0.6 m monochromator with a resolution of 0.3 nm. To the best of our knowledge, the wavelength-resolved emission spectra of CH3S for the 312 and 201310 bands are being reported here for the first time and these provide new vibrational intervals hitherto unobserved. Fundamental vibrational frequencies have been determined for the nu2 (umbrella) and nu3 (C-S stretch) modes of CH3S for both the excited A 2A1 state and the ground X 2E state: nu'2=1096 cm-1, nu'3=402 cm-1, nu2=1328 cm-1 and nu3=742 cm-1. In addition, the nu6 (rocking) mode vibrational frequency has been obtained for the A 2A1 state, i.e. nu'6=628 cm-1. Vibrational and anharmonic parameters have been determined by a least-squares fit of the progressions involving the symmetric nu3 vibration in the A 2A1 - X 2E3/2 excitation spectra and involving both the nu2 and nu3 progressions in the dispersed fluorescence spectra.

Laser excitation and emission spectroscopy of the methoxy radical in a supersonic jet

Laser-induced excitation and wavelength-resolved emission spectra of the methoxy (CH3O) radical have been obtained in a supersonic jet environment. Fluorescence in the near ultraviolet from several vibronic bands belonging to the A2A1-X2E electronic system of CH3O has been dispersed by a 0.6 m monochromator with a resolution of 0.3 nm. A complete set of vibrational frequencies with all assignments for the X2E state of CH3O has been obtained. To the best of our knowledge, the dispersed spectra of CH3O for the 220 and 210310 bands are presented here for the first time. The vibrational constants determined by a least-squares fit for the ν3 mode are ω″e = 1071 cm−1 and ω″ex″e = 8.4 cm−1. For the Jahn-Teller active ν6 mode, the vibrational and anharmonic constants ω″e = 786 cm−1 and ω″ex″e = 55 cm−1, respectively, have been obtained for the first time.