Laser-induced fluorescence in N2 and N+2 by multiple-photon excitation at 266 nm

The fluorescence transitions corresponding to the second positive system of N₂ (C³Π_u → B³Π_g) for Δv = 0, 1 and the first negative system of N⁺₂(B²Σ⁺_u → X²Σ⁺_g) for Δv = 0, 1, 2 have been observed following laser-induced mul excitation of N₂.     

Chemiluminescent emissions from the gas phase reactions of ozone with acetylene and allene

Chemiluminescence spectra (300–800 nm) from the reactions of ozone with acetylene and allene have been obtained. These spectra show the production of electronically excited CHO, OH(²Π_i, υ ? 9) and possibly C₂(B³Π_g, υ′ = 0 → X³Π_u, υ″ = 6) from the O₃ + C₂H₂ reaction. CH(²Δ), OH(²Σ⁺) and OH(²Π_j, υ ? 9) emissions were identified from the O₃ + C₃H₄ reaction in addition to the CH₂O(¹A″) emission previously reported.     

Investigation of the molecular chemiluminescence SrCl(A2Π1/2,3/2, B2Σ+ → X2Σ+) and the atomic resonance fluorescence Sr(53P1 → 51S0) in the time-domain following the pulsed dye laser generation of Sr(53 PJ) in the presence of CH2Cl2

Time-resolved investigations of the atomic resonance fluorescence Sr(5³P₁ → 5¹S₀) and the molecular chemiluminescence from SrCl(A²Π_1/2,3/2, B²Σ⁺ → X²Σ⁺) are reported following the reaction of the electronically excited strontium atom, Sr(5s5p(³P_J)), 1.807 eV above its 5s²(¹S₀) electronic ground state, with CH₂Cl₂. The optically metastable strontium atom was generated by pulsed dye-laser excitation of ground state strontium vapor to the Sr(5³P₁) state at λ = 689.3 nm (Sr(5³P₁ ← 5¹S₀)) at elevated temperature (850 K) in the presence of excess helium buffer gas in which rapid Boltzmann equilibration within the 5³P_J manifold takes place. Sr(5³P_J) was then monitored by time-resolved atomic fluorescence from Sr(5³P₁) at the resonance wavelength together with chemiluminescence from electronically excited SrCl resulting from reaction of the excited atom with CH₂Cl₂. The molecular systems recorded in the time-domain were SrCl(A²Π_1/2 → X²Σ⁺) (Δν = 0, λ = 674 nm), SrCl(A²Π_3/2 → X²Σ⁺) (Δν = 0, λ = 660 nm), and SrCl(B²Σ⁺ → X²Σ⁺) (Δν = 0, λ = 636 nm). Both the A²Π (179.0 kJ mol^?1) and (B²Σ⁺(188.0) kJ mol^?1) states of SrCl are energetically accessible on collision between Sr(³P) and CH₂Cl₂. Exponential decay profiles for both the atomic and molecular (A,B – X) chemiluminescence emission are observed and the first-order decay coefficients characterized in each case. These are found to be equal under identical conditions and hence SrCl(A²Π, B²Σ⁺) are shown to arise from direct Cl-atom abstractions on reaction with this halogenated species. The combination of integrated molecular and atomic intensity measurements, coupled with optical sensitivity calibration, yields estimations of the branching ratios into the A_1/2,3/2, B, and X states arising from Sr(5³ P_J) + CH₂Cl₂ which are found to be as follows: A_1/2, 3.0 × 10^?3; A_3/2, 1.7 × 10^?3; B, 4.4 × 10^?4 yielding ΣSrCl(A_1/2 + A_3/2 + B) = 5.1 × 10^?3. As only the X, A and B states of SrCl are accessible on reaction, this indicates an upper limit for the branching ratio into the ground state of 0.995. The present results are compared with previous time-resolved measurements on SrF, Cl, Br(A²Π,B²Σ⁺ ? X²Σ⁺) that we have reported on various halogenated species and with analogous chemiluminescence studies on Sr(³P) with other halides obtained from molecular beam measurements. The results are further compared with those from a series of previous analogous investigations in the time-domain we have presented of molecular emissions from CaF, Cl, Br, I (A,B – X) arising from the collisions of Ca(4³P_J) with appropriate halides and with branching ratio data for Ca(4³P_J) obtained in beam measurements. © 1995 John Wiley & Sons, Inc.     

The rotational excitation and population distribution of OH(A2Σ+) produced by electron impact on water

The rotational excitation and population distributions in OH(A²Σ⁺, υ′ = 0) have been determined by analyzing the OH(A²Σ⁺ → X²Π_i) emission spectrum. The spectrum results from the impact of mon-energetic electrons (0–100 eV) on water vapour. It is shown that these rotational distributions of the OH(A²Σ⁺) state depend on the electron impact energy and have contributions from singlet and triplet states of water. The contribution from each dissociative state of water can be described by a Boltzmann distribution, both in the case of rotational excitation and population.Three distribution parameters (“temperatures”) for rotational excitation are obtained, namely 13800 K and 2900 K for the singlet contributions and 4000 K for the triplet contribution. The corresponding distribution parameters for the rotational population are 30000, 3300, and 4800 K, respectively. The results are discussed in view of recent theoretical calculations on water energy levels.     

Absolute emission cross sections for the calibration of optical detection systems in the 120–250 nm range

Absolute emission cross sections have been determined for electron impact on CO, NO and N₂. For the CO(A ¹ΠX ¹Σ⁺) and N₂(a ¹ΠX ¹Σ_g) radiation our data is in good agreement with that of other groups. For CO⁺ (B²Σ⁺X²Σ⁺) the values of the emission cross sections are different from those measured previously. This discrepancy is explained in terms of an inadequate straylight correction in the former experiments. For the NO(A²Σ⁺X²Π) emission no previous σ_em values are known to the authors. Furthermore the electronic transition moments of the NO(A²Σ⁺X²Π) and CO⁺(B²ΣX²Σ⁺) systems have been measured and are found to be independent of the internuclear distance.     

O‐loss photodissociation of the OCS+ ion in the low‐lying electronic states studied using multiconfiguration second‐order perturbation theory

The CASPT2 potential energy curves (PECs) for O‐loss dissociation from the X²Π, A²Π, B²Σ⁺, C²Σ⁺, 1⁴Σ^?, 1²Σ^?, and 1⁴Π states of the OCS⁺ ion were calculated. The PEC calculations indicate that X²Π, 1⁴Σ^?, 1²Σ^?, and 1⁴Π correlate with CS⁺(X²Σ⁺) + O(³P_g); A²Π and B²Σ⁺ correlate with CS⁺(A²Π) + O(³P_g); and C²Σ⁺ probably correlates with CS⁺(X²Σ⁺) + O(¹D_g). The CASSCF minimum energy crossing point (MECP) calculations were performed for the C²Σ⁺/1⁴Σ^?, C²Σ⁺/1⁴Π, A²Π/1⁴Σ^?, A²Π/1²Σ^?, A²Π/1⁴Π, and B²Σ⁺/1²Σ^? state pairs and the spin‐obit couplings were calculated at the located MECPs. A conical intersection point between the B²Σ⁺ and C²Σ⁺ potential energy surfaces was found at the CASSCF level. Based on our calculations, seven O‐loss predissociation processes of the C²Σ⁺ state are suggested and an appearance potential value of 7.13 eV for the CS⁺ + O product group is predicted. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Energy transfer processes in reactions of He(2 3S) and Ne(3P0,2) with CS2

A comparative spectroscopic study in the visible and ultraviolet ranges was conducted on the flowing afterglows resulting from the reactions of He(2 ³S) and Ne(³P_0,2) metastables with CS₂. Penning ionization was found to be the predominant energy transfer process. However, electron—ion recombination within the afterglows constitutes a major secondary process and gives rise to the most intense emitting system, CS(A ¹ Π → X ¹Σ⁺). Both afterglows were found to produce the CS⁺₂($\text{B}$²Σ⁺_u-$\text{X}$²Π_g), CS⁺₂($\text{A}$²Π_u-$\text{X}$²Π_g) and CS(a ³Π-X ¹Σ⁺) emission systems as well as some atomic sulfur emission lines. Some intensity differences were observed and are interpreted in terms of energetics and the formation mechanisms of the emitting species. A moderately strong CS⁺(A ²Π_i-X ²Σ⁺) emission system was also observed in the ehlium afterglow. In addition, a weak, sharp group of bands in the 390–420 nm range in the helium afterglow has been determined to be due to the presence of a small amount of He⁺ ions. This group of bands consists of two overlapping emission systems and are identified as CS(B ¹Σ⁺ → A ¹Π) and CS⁺(B ²Σ⁺ → A ²Π_i).     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

CS+(B2Σ+ → A 2Πi) fluorescence from penning excitation of CS

The energy transfer reation of He(2³S) + CS was studied spectroscopically in a flowing afterglow apparatus. The CS⁺(B²Σ⁺ → A ²Π_i) transition is identified via three members of the Δν = 0 sequence (406–415 nm). The spin-orbit splitting of the (0, 0) band of CS⁺(A ²Π_i) is 301 ± 5 cm^?1. A weak emitting system (280–340 nm) is tentatively identified as CS⁺(B²Σ⁺→ X²Σ⁺).     

Detection of nitrogen rotational distributions by resonant 2 + 2 multiphoton ionization through the a1Πg state

Characterization of laser 2 + 2 multiphoton ionization of nitrogen to obtain rotational state distributions has been investigated via the resonant two-photon transition a ¹Π_g(ν = 1) ← X ¹Σ_g(ν = 0). For room-temperature nitrogen, the spectral intensities and state distribution are directly related and give rotational temperatures of 290 ± 20 K. For power densities of 3 GW/cm², the ionization probability is 1 × 10⁻⁵ per N₂ molecule per average rotational state.     

Photoexcitation of CH3NCO,CH3NCS and CH3SCN in the vacuum ultraviolet: Rydberg states and photofragment emission

Photoabsorption cross sections and fluorescence excitation spectra of CH₃NCO, CH₃NCS and CH₃SCN vapor were measured in the vacuum ultraviolet using synchrotron radiation. Many sharp structures observed from CH₃NCO and CH₃SCN in the 120–180 nm region are classified into three Rydberg series and their vibrational progressions, whereas for CH₃NCS six broad bands exhibit no fine structure. The emission which starts to appear at 172.8 ± 1.0 nm excitation of CH₃NCO is attributed to the NCO(A²Σ⁺-X²Π) band. The emissions from CH₃NCS and CH₃SCN are assigned to the A²Π-X²Π and B²Σ⁺-X²Π bands of NCS; the CN(B²Σ⁺-X²Σ⁺) band is also observed at 125 nm excitation of CH₃SCN. The photodissociation processes are discussed in accord with the emission observed.     

Fluorescence and recombination-emission spectra from S2 formed by ultraviolet laser photolysis of CS2 in an argon matrix at 15 k

Two series of emission bands were observed for the CS₂/Ar(1 : 100–500) system at 15 K with excitation at 257.3 nm. They are assigned to B³Σ^?_u → χ³Σ^?_g and B″³Π_u → X³Σ^?_g of S₂, which was formed by photodissociation of CS₂, CS₂ + hv → CS + S, followed by recombination of two S atoms. The B″³Π_u state has been found 524 cm^-1 lower in energy than B³Σ^?_u     

Sensitization of the CN(B2Σ+ → X2Σ+) emission by Hg(63P0) metastables

The vibrational analysis of the CN(B²Σ⁺ → X²Σ⁺) emission sensitized by Hg(6³P₀) metastables has shown that the energy transfer process, Hg(6³P₀) + CN(X²Σ⁺) → Hg(6¹S₀) + CN(B²Σ⁺), populates the CN(B²Σ⁺) state in a non-Franck-Condon fashion. The relative vibrational populations for the ν = 0 to 4 states are 1.00, 0.56 ± 0.06, 0.26 ± 0.03, 0.11 ± 0.03 and 0.04 ± 0.01, respectively. Long-range attractive interaction between the Hg(6³P₀) atom and the CN(X²Σ⁺) radical is evidenced by the observed high rotational excitation of the CN(B²Σ⁺) radical following the energy transfer process.     

Vibrational energy transfer from N2 to CO on electron excitation of N2-CO solids at 4.2 K

Spectra emitted from 0.1% CO-N₂ solids excited with high energy electrons at 4 K show evidence for resonant transfer of vibrational energy from highly excited vibrational levels of N₂ to CO in the process N₂(X¹Σ_g⁺, ν) + CO(ν = 0) → N₂(X¹Σ_g⁺, ν - 1) + CO(ν = 1) + phonons. Energy transfer from levels with ν ? 9 has been observed.     

C2 and CN formation by optical pumping of CO/Ar and Co/N2/Ar mixtures at room temperature

A continuous wave carbon monoxide laser is used to excite the vibrational mode of CO in CO/Ar and CO/N₂/Ar mixtures flowing through a gas absorption cell. High steady-state excitation of the CO vibrational mode (0.3 eV/molecule) is achieved, while a translational—rotational temperature near 300 K is maintained by the steady flow of cold gas into the cell. These non-equilibrium conditions result in extreme vibration—vibration pumping, population high-lying vibrational quantum levels (to V = 42) of CO. N₂ can also be pumped by vibrational energy transfer from CO. Under these conditions, C₂ and CN molecules are formed, and are observed to fluoresce on various electronic band transitions, notably C₂ Swan (A ³Π_g—X ³Π_u) and CN violet (B ²Σ⁺—X²Σ⁺).     

Sensitization of the OH(A 2Σ+ → X 2Πi) emission by Hg(6 3P0) metastables

The electronic energy transfer process Hg(6 ³P₀) + OH(X²Π_i, υ = 0,K) → Hg(6 ¹S₀) + OH(A ²Σ⁺, υ,K) has been studied by the sensitized fluorescence method. A rather broad spectrum of rotational population, N_υ′K′, was obtained under conditions of minimum relaxation, which illustrates the non-resonant and non-optical nature of this energy transfer process. The fractions of the exoergicity, above electronic excitation of OH(A ²Σ⁺, υ = 0, K = 0), going into vibrational, rotational and translational excitation are 0.11, 0.31, and 0.58, respectively. A statistical mode of energy partitioning, such as would result from long-lived complex formation, seems to account well for these observations.     

Electronic,spectra, and spin orbit interaction for FrAr van der Waals system

The potential energy curves and spectroscopic constants of the ground and many excited states of the FrAr van der Waals system have been determined using a one‐electron pseudopotential approach. The Fr⁺ core and the electron–Ar interactions are replaced by effective potentials. The Fr⁺Ar core–core interaction is incorporated using the accurate CCSD(T) potential of Hickling et al. (Phys. Chem. Chem. Phys. 2004, 6, 4233). This approach reduces the number of active electrons of the FrAr van der Waals system to only one valence electron, which permits the use of very large basis sets for the Fr and Ar atoms. Using this technique, the potential energy curves of the ground and many excited states are calculated at the self consistent field (SCF) level. In addition, the spin–orbit interaction is also considered using the semiempirical scheme for the states dissociating into Fr (7p) and Fr (8p). The FrAr system is not studied previously and its potential interactions, spectroscopic constants and dipole functions are presented here for the first time. Furthermore, we have predicted the X²Σ⁺–A²Π_1/2, X²Σ⁺–AΠ_3/2, X²Σ⁺–B²Σ_1/2⁺, X²Σ⁺–3²Π_1/2, X²Σ⁺–3²Π_3/2, and X²Σ⁺–5²Σ_1/2⁺ absorption spectra. © 2012 Wiley Periodicals, Inc.     

Multireference configuration interaction study on spectroscopic parameters and molecular constants of PO and PO+

The highly accurate valence internally contracted multireference configuration interaction (MRCI) approach has been employed to investigate the potential energy curves (PECs) for the X²Π, b⁴Σ^?, C²Σ^? states of PO and the X¹Σ⁺ state of PO⁺. For these electronic states, the spectroscopic parameters of the isotopes (P¹⁶O, P¹⁸O, P¹⁶O⁺, and P¹⁸O⁺) have been determined and compared with those of the investigations reported in the literature. The comparison shows that excellent agreement exists between the present results and the available experiments. With the PECs determined here, the first 30 vibrational states for P¹⁶O(X²Π, b⁴Σ^?), P¹⁸O(X²Π, b⁴Σ^?), P¹⁶O⁺(X¹Σ⁺), and P¹⁸O⁺(X¹Σ⁺) are computed when the rotational quantum number J equals zero (J = 0). The vibrational level G(υ), inertial rotation constant B_υ and centrifugal distortion constant D_υ are determined when J = 0. All the results of vibrational states except for P¹⁶O (X²Π) are reported for the first time. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Fluorescence spectra of laser-excited YO molecules in a H2O2Ar flame

CW dye laser induced fluorescence emission and thermal emission spectra of YO-molecules in a 1 atm H₂O₂Ar flame of 2430 K were recorded simultaneously. Narrow band laser excitation was applied to four rotational lines in the (1, 1) Q-branch of the A²Π_{$\text{3}\text{2}$}→X²Σ⁺ transition and broadband excitation was applied to several separate Q-branches of the A²Π_{$\text{1}\text{2}$,$\text{3}\text{2}$}→X²Σ⁺ transitions. From the differences between the fluorescence emission spectra and thermal emission spectra, we conclude that collisional de-excitation of an excited vibronic level takes place by vibrational relaxation, decay to the electronic ground state and by intermultiplet transfer in order of increasing probability.     

Rotational analysis of the a3Π0+,1-X1Σ+ emission systems of GeH+

New emission systems have been observed from the helium afterglow reaction of GeH₄ in the 520–610 nm region. On the basis of the rotational analysis, they were assigned to the a ³Π_0⁺-X¹Σ⁺ and a³Π₁-X¹Σ⁺ subsystems of GeH⁺. Spectroscopic constants have been determined for the GeH⁺ (a³Π_0⁺, a³Π₁, X¹Σ⁺) states.