Observation and analysis of the 2g(1D) ion-pair state of I2: the g/u mixing between the 1u(1D) and 2g(1D) states

We report the analysis of the 2g(1D) ion-pair state of I2 by perturbation-facilitated optical-optical double resonance. The present study began with the observation of the 2g(1D)-A' 3Pi(2u) emission at around 230 nm during the analysis of the ultraviolet emissions originating form the 1u(1D) ion-pair state. The identification of this new transition helped us to specify the wavelengths for detecting the 2g(1D) state by emission, and also to estimate its absolute position. The intermediate states used to observe the 2g(1D) state were the B 3Pi(0u(+))-b' 2u mixed states by the hyperfine interaction, which allowed us to combine the X 1Sigmag(+) ground state with the 2g(1D) state in the (1+1) photon excitation following the optical selection rules for one-photon transitions: 2g(1D)<--b' 2u-B 3Pi(0u(+))<--X 1Sigmag(+). Our analysis covered the 2g(1D) state in the 0< or =v< or =12 and 9< or =J< or =40 ranges. The molecular constants and Rydberg-Klein-Rees (RKR) potential of the 2g(1D) state were reported. We discussed the occurrence of the 2g(1D)-A' 3Pi(2u) emission, when exciting to the 1u(1D) v=0 state, and attributed it to the g/u mixing between the 2g(1D) and 1u(1D) states by the hyperfine interaction. The effect of the perturbation on measured line intensities and lifetimes was evident.     

Energy transfer in the d3Pi(g)-a3Pi(u) (0-0) Swan bands of C2: implications for quantitative measurements

Energy transfer effects on dicarbon (C2) d3Pi(g) <-- a3Pi(u) laser-induced fluorescence (LIF) spectra in fuel-rich acetylene atmospheric-pressure flames have been studied using a combination of two different two-dimensional techniques. Measurements using a picosecond laser system in conjunction with a streak camera allowed determination of typical fluorescence lifetimes of levels in the d state and of population changes introduced by rotational energy transfer (RET) and by state-dependent quenching. Excitation-emission spectroscopy yielded two-dimensional maps containing all excitation and all emission spectra in the spectral ranges between 19 340 and 20 150 cm(-1) (excitation) and from 546 to 573 nm (emission) and allowed unambiguous assignment of all transitions in this spectral region. Our measurements show a comparatively long quenching lifetime (around 2 ns) and dominant effects of energy transfer on shape and intensity of the acquired spectra (90% of the fluorescence stems from levels populated by ET). A pronounced dependence of the total RET on the quantum number of the initially excited state is observed. Vibrational energy transfer (VET) is significantly weaker (only 5% contribution for excitation in the v' = 1 level). Implications for quantitative concentration measurements are discussed, and exemplary spatially resolved profiles in a well-characterized low-pressure propene flame are presented.     

Observation of the d3Pi(g)<--c3Sigma(u)+ band system of C2

A new band system of C(2), d (3)Pi(g)<--c (3)Sigma(u) (+) is observed by laser induced fluorescence spectroscopy, constituting the first direct detection of the c (3)Sigma(u) (+) state of C(2). Observations were made by laser excitation of c (3)Sigma(u) (+)(v(")=0) C(2), produced in an acetylene discharge, to the d (3)Pi(g)(v(')=3) level, followed by detection of Swan band fluorescence. Rotational analysis of this band yielded rotational constants for the c (3)Sigma(u) (+)(v(")=0) state: B(0)=1.9218(2) cm(-1), lambda(0)=-0.335(4) cm(-1) and gamma(0)=0.011(2) cm(-1). The vibrational band origin was determined to be nu(3-0)=15861.28 cm(-1).     

The NaK 1(b) 3Pi(Omega=0) state hyperfine structure and the 1(b) 3Pi(Omega=0) approximately 2(A) 1Sigma+ spin-orbit interaction

We have measured the hyperfine structure of mutually perturbing rovibrational levels of the 1(b) 3Pi0 and 2(A) 1Sigma+ states of the NaK molecule, using the perturbation-facilitated optical-optical double resonance method with copropagating lasers. The unperturbed 1(b) 3Pi0 levels are split into four hyperfine components by the Fermi contact interaction bFIS. Mixing between the 1(b) 3Pi0 and 2(A) 1Sigma+ levels imparts hyperfine structure to the nominally singlet component of the perturbed levels and reduces the hyperfine splitting of the nominally triplet component. Theoretical analysis relates these observations to the hyperfine splitting that each 1(b) 3Pi0 level would have if it were not perturbed by a 2(A) 1Sigma+ level. Using this analysis, we demonstrate that significant hyperfine splitting arises because the 1(b) 3Pi0 state cannot be described as pure Hund's case (a). We determine bF for the 1(b) 3Pi0 levels and also a more accurate value for the magnitude of the singlet-triplet spin-orbit coupling HSO=[1(b) 3Pi0(vb,J)(H(SO))2(A) 1Sigma+(vA,J). Using the known spectroscopic constants of the 1(b) 3Pi state, we obtain bF=0.009 89+/-0.000 27 cm(-1). The values of (H(SO)) are found to be between 2 and 3 cm(-1), depending on vb, vA, and J. Dividing (H(SO)) by calculated vibrational overlap integrals, and taking account of the 1(b) 3Pi(Omega) rotational mixing, we can determine the magnitude of the electronic part H(el) of H(SO). Our results yield (H(el))=(16.33+/-0.15) cm(-1), consistent with our previous determinations using different techniques.     

Electric-field-induced g/u mixing of the E0g+(3P2) and D0u+(3P2) ion-pair states of jet-cooled I2 observed using optical triple resonance

Electric-field-induced electronic state g/u mixing of nearly isoenergetic rovibrational levels of the E0g+(3P2) and D0u+(3P2) ion-pair states of I2 has been observed using optical triple resonance combined with resonance ionization. Detectable mixing with applied fields of 1 kV/cm occurs over a range of energy level separations of < or = 0.3 cm(-1).     

Fluorescence excitation spectra of the b (1)Pi(u), b(') (1)Sigma(u) (+), c(n) (1)Pi(u), and c(n) (') (1)Sigma(u) (+) states of N(2) in the 80-100 nm region

Fluorescence excitation spectra produced through photoexcitation of N(2) using synchrotron radiation in the spectral region between 80 and 100 nm have been studied. Two broadband detectors were employed to simultaneously monitor fluorescence in the 115-320 nm and 300-700 nm regions, respectively. The peaks in the vacuum ultraviolet fluorescence excitation spectra are found to correspond to excitation of absorption transitions from the ground electronic state to the b (1)Pi(u), b(') (1)Sigma(u) (+), c(n) (1)Pi(u) (with n=4-8), c(n) (') (1)Sigma(u) (+) (with n=5-9), and c(4) (')(v('))(1)Sigma(u) (+) (with v(')=0-8) states of N(2). The relative fluorescence production cross sections for the observed peaks are determined. No fluorescence has been produced through excitation of the most dominating absorption features of the b-X transition except for the (1,0), (5,0), (6,0), and (7,0) bands, in excellent agreement with recent lifetime measurements and theoretical calculations. Fluorescence peaks, which correlate with the long vibrational progressions of the c(4) (') (1)Sigma(u) (+) (with v(')=0-8) and the b(') (1)Sigma(u) (+) (with v(') up to 19), have been observed. The present results provide important information for further unraveling of complicated and intriguing interactions among the excited electronic states of N(2). Furthermore, solar photon excitation of N(2) leading to the production of c(4) (')(0) may provide useful data required for evaluating and analyzing dayglow models relevant to the interpretation of c(4) (')(0) in the atmospheres of Earth, Jupiter, Saturn, Titan, and Triton.     

The d (3)Pi(g)-c (3)Sigma(u) (+) band system of C2

A two-dimensional fluorescence (excitation/emission) spectrum of C2 produced in an acetylene discharge was used to identify and separate emission bands from the d (3)Pi(g)<--c (3)Sigma(u) (+) and d (3)Pi(g)<--a (3)Pi(u) excitations. Rotationally resolved excitation spectra of the (4<--1), (5<--1), (5<--2), and (7<--3) bands in the d (3)Pi(g)<--c (3)Sigma(u) (+) system of C2 were observed by laser-induced fluorescence spectroscopy. The molecular constants of each vibrational level, determined from rotational analysis, were used to calculate the spectroscopic constants of the c (3)Sigma(u) (+) state. The principal molecular constants for the c (3)Sigma(u) (+) state are B(e)=1.9319(19) cm(-1), alpha(e)=0.018 55(69) cm(-1), omega(e)=2061.9 cm(-1), omega(e)x(e)=14.84 cm(-1), and T(0)(c-a)=8662.925(3) cm(-1). We report also the first experimental observations of dispersed fluorescence from the d (3)Pi(g) state to the c (3)Sigma(u) (+) state, namely, d (3)Pi(g)(v=3)-->c (3)Sigma(u) (+)(v=0,1).     

Evidence of a weak dipole transition moment between the X(1)0g+ and (3)1u(5(3)P1) electronic energy states in Cd2

Theoretical and experimental evidence of a weak M(z)(R) dipole transition moment between the X(1)0g+ ground and (3)1u(5(3)P1) excited states in Cd2 is presented. Two independent attempts at recording an excitation spectrum of the (3)1u <-- X(1)0g+ transition using a laser beam crossed with a supersonic free-jet expansion beam are reported. The measurements were performed in a spectral range predicted as a result of both ab initio calculations of the electronic energy-state potentials involved in the transition and a simulation of the excitation spectrum. Unfortunately it was impossible to provide unambiguous experimental support for the calculated (3)1u-state potential, due to the very poor signal to noise ratio. However, the experimental results corroborate the very small values of the <(3)1u|M(z)|X(1)0g+> elements obtained in the calculations. This work provides as a reliable starting point for further analysis of the (3)1u-state characteristics.     

Predissociation mechanism for the lowest 1 Pi u states of N2

Separate coupled-channel Schr?dinger-equation (CSE) models of the interacting (1)Pi(u) (b,c,o) and (3)Pi(u) (C,C(')) states of N(2) are combined, through the inclusion of spin-orbit interactions, to produce a five-channel CSE model of the N(2) predissociation. Comparison of the model calculations with an experimental database, consisting principally of detailed new measurements of the vibrational and isotopic dependence of the (1)Pi(u) linewidths and lifetimes, provides convincing evidence that the predissociation of the lowest (1)Pi(u) levels in N(2) is primarily an indirect process, involving spin-orbit coupling between the b (1)Pi(u)- and C (3)Pi(u)-state levels, the latter levels themselves heavily predissociated electrostatically by the C(') (3)Pi(u) continuum. The well-known large width of the b(v=3) level in (14)N(2) is caused by an accidental degeneracy with C(v=9). This CSE model provides the first quantitative explanation of the predissociation mechanism for the dipole-accessible (1)Pi(u) states of N(2), and is thus likely to prove useful in the construction of realistic radiative-transfer and photochemical models for nitrogen-rich planetary atmospheres.     

Characterization of a shallow-bound 0g+ valence state of I2 using emission from the D 0u+(3P2) and F' 0u+(1D2) ion-pair states populated by amplified spontaneous emission

A range of vibrational levels of the D 0(u)(+)((3)P(2)) and F' 0(u)(+)((1)D(2)) ion-pair states of I(2) is shown to be easily generated by amplified spontaneous emission (ASE) from their more accessible partners, E 0(g)(+)((3)P(2)) and f' 0(g)(+)((1)D(2)), in sufficient concentration for dispersed fluorescence studies of the D 0(u)(+)((3)P(2)) --> 0(g)(+)(bb) and F' 0(u)(+)((1)D(2)) --> 0(g)(+)(bb) transitions to be carried out. T(0) (J = 49) of this shallow-bound 0(g)(+)(bb) valence state is unambiguously determined and an improved R(e) value of 3.952 +/- 0.005 A is obtained from optimizing the fit of the intensities of the vibrational progressions in the 0(g)(+)(bb) state, and T(e) is found to be 27311.3 +/- 2 cm(-1), leading to D(e) = 442.0 +/- 2 cm(-1).     

Interactions of the 3ppi u c1Pi u(v=2) Rydberg-complex member in isotopic N2

The 3ppi u c1Pi u-X 1Sigmag+(2,0) Rydberg and b' 1Sigmau+-X 1Sigmag+(7,0) valence transitions of 14N2, 14N15N, and 15N2 are studied using laser-based 1 extreme ultraviolet (XUV)+1' UV two-photon-ionization spectroscopy, supplemented by synchrotron-based hotoabsorption measurements in the case of 14N2. For each isotopomer, effective rotational interactions between the c(v=2) and b'(v=7) levels are found to cause strong Lambda-doubling in c(v=2) and dramatic P/R-branch intensity anomalies in the b'-X(7,0) band due to the effects of quantum interference. Local perturbations in energy and predissociation line width for the c(v=2) Rydberg level are observed and attributed to a spin-orbit interaction with the crossing, short-lived C 3Pi u(v=17) valence level.     

Non-adiabatic transitions from I2(E0g+ and D0u+) states induced by collisions with M = I2(X0g+) and H2O

The stepwise two-step two-color and three-step three-color laser excitation schemes are used for selective population of rovibronic levels of the first-tier ion-pair E0(g)(+) and D0(u)(+) states of molecular iodine and studies of non-adiabatic transitions to the D and E states induced by collisions with M = I(2)(X) and H(2)O. Collection and analysis of the luminescence after excitation of the v(E) = 8, 13 and v(D) = 13, 18 vibronic levels of the E and D states in the pure iodine vapor and the gas-phase mixtures with H(2)O provide rate constants for the non-adiabatic transitions to the D and E state induced by collisions with these molecules. Vibrational distributions for the [formula: see text] collision-induced non-adiabatic transitions (CINATs) are obtained. Rather strong λ(lum)(max) ≈ 3400 ? luminescence band is observed in the I(2) + H(2)O mixtures, whereas its intensity is ~100 times less in pure iodine vapor. Radiative lifetimes and quenching rate constants of the I(2)(E,v(E) = 8, 13 and D,v(D) = 13, 18) vibronic state are also determined. Rate constants of the [formula: see text], v(E) = 8-54, CINATs are measured again and compared with those obtained earlier. New data confirm resonance characters of the CINATs found in our laboratory about 10 years ago. Possible reasons of differences between rate constant values obtained in this and earlier works are discussed. It is shown, in particular, that differences in rate constants of non-resonant CINATs are due to admixture of water vapor in iodine.     

The rotational spectrum of the NiI radical in the X 2Delta(5/2) and A 2Pi(3/2) states

The millimeter- and submillimeter-wave spectra of the NiI radical in the X (2)Delta(5/2) and A (2)Pi(3/2) states were observed by a source-modulated microwave spectrometer. The NiI radical was generated by a dc glow discharge in the mixture of CH(3)I vapor and Ar gas through the sputtering reaction with a Ni cathode. Observed transition frequencies for each electronic state were independently analyzed using a polynomial energy expression based on Hund's case (c) approximation. The deperturbed rotational constants were also estimated by the perturbation analysis including interaction terms between the ground state and the lowest excited state.     

Isotopic variation of experimental lifetimes for the lowest 1 Pi u states of N2

Lifetimes of several (1)Pi(u) states of the three natural isotopomers of molecular nitrogen, (14)N(2), (14)N(15)N, and (15)N(2), are determined via linewidth measurements in the frequency domain. Extreme ultraviolet (XUV)+UV two-photon ionization spectra of the b (1)Pi(u)(v=0-1,5-7) and c(3) (1)Pi(u)(v=0) states of (14)N(2), b (1)Pi(u)(v=0-1,5-6) and c(3) (1)Pi(u)(v=0) states of (14)N(15)N, and b (1)Pi(u)(v=0-7), c(3) (1)Pi(u)(v=0), and o (1)Pi(u)(v=0) states of (15)N(2) are recorded at ultrahigh resolution, using a narrow band tunable XUV-laser source. Lifetimes are derived from the linewidths of single rotationally resolved spectral lines after deconvolution of the instrument function. The observed lifetimes depend on the vibrational quantum number and are found to be strongly isotope dependent.     

High resolution vacuum ultraviolet emission spectrum of D(2) from 78 to 103 nm: The D (1)Pi(u)-->X (1)Sigma(g) (+) and D(') (1)Pi(u) (-)-->X (1)Sigma(g) (+) band systems

The emission spectrum of the D(2) molecule has been studied at high resolution in the vacuum ultraviolet region 78.5-102.7 nm. A detailed analysis of the two D (1)Pi(u)-->X (1)Sigma(g) (+) and D(') (1)Pi(u) (-)-->X (1)Sigma(g) (+) electronic band systems is reported. New and improved values of the level energies of the two upper states have been derived with the help of the program IDEN [V. I. Azarov, Phys. Scr. 44, 528 (1991); 48, 656 (1993)], originally developed for atomic spectral analysis. A detailed comparison is made between the observed energy levels and solutions of coupled equations using the newest ab initio potentials by Wolniewicz and co-workers [J. Chem. Phys. 103, 1792 (1995); 99, 1851 (1993); J. Mol. Spectros. 212, 208 (2002); 220, 45 (2003)] taking into account the nonadiabatic coupling terms for the D (1)Pi(u) state with the lowest electronic states B (1)Sigma(u) (+), C (1)Pi(u), and B(') (1)Sigma(u) (+). A satisfactory agreement has been found for most of the level energies belonging to the D and D(') states. The remaining differences between observation and theory are probably due to nonadiabatic couplings with other higher electronic states which were neglected in the calculations.     

The K2 2(3)Pi(g) state: new observations and analysis

Totally 3045 transitions into the 2(3)Pi(g) v = 0-42, J = 0-103, Omega = 0, 1, 2 rovibrational levels have been observed by infrared-infrared double resonance fluorescence excitation and two-photon spectroscopy. Molecular constants including the spin-orbit interaction parameters are obtained. Although the K2 2(3)Pi(g) state dissociates to the 4s + 3d atomic limit, it is strongly mixed with the 3P ionic states in the range of the potential well. This mixing results in a relatively large equilibrium internuclear distance Re = 5.254 A and a larger spin-orbit constant A0 approximately 14.17 cm(-1) than that of the atomic limit -2.33 cm(-1). Strong perturbations of the 2(3)Pi(g) levels observed are attributed to the spin-orbit coupling with the 4(1)Sigma(g)+ state.     

Hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2)

The hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2) have been resolved with sub-Doppler continuous wave perturbation facilitated optical-optical double resonance spectroscopy via A (1)Sigma(u) (+) approximately b (3)Pi(u) mixed intermediate levels. The hyperfine patterns of these three states are similar. The hyperfine splittings of the low rotational levels are all very close to the case b(betaS) limit. As the rotational quantum number increases, the hyperfine splittings become more complicated and the coupling cases become intermediate between cases b(betaS) and b(beta J) due to spin-rotation interaction. We present a detailed analysis of the hyperfine structures of these three (3)Sigma(g) (+) states, employing both case b(betaS) and b(beta J) coupling basis sets. The results show that the hyperfine splittings of the (3)Sigma(g) (+) states are mainly due to the Fermi-contact interaction. The Fermi contact constants for the two d sigma Rydberg states, the 2 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+), are 245+/-5 MHz and 225+/-5 MHz, respectively, while the Fermi contact constant of the s sigma 3 (3)Sigma(g) (+) Rydberg state is 210+/-5 MHz. The diagonal spin-spin and spin-rotation constants, and nuclear spin-electronic spin dipolar interaction parameters of the 3 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+) states are also obtained.     

The [1+1] two-photon dissociation spectra of CO2 + via A 2Pi u,12(upsilon1upsilon20)<--X 2Pi g,12(000) transitions

In the wavelength range of 235-354 nm, we have obtained the mass-resolved [1+1] two-photon dissociation spectra of CO(2) (+) via A (2)Pi(u,12)(upsilon(1)upsilon(2)0)<--X (2)Pi(g,12)(000) transitions by preparing CO(2) (+) ions in the X (2)Pi(g,12)(000) state via [3+1] multiphoton ionization of CO(2) molecules at 333.06 nm. The vibronic bands of (upsilon(1)20;upsilon(1)=0-11)micro (2)Pi(12) and (upsilon(1)20;upsilon(1)=0-6)kappa (2)Pi(12) involving the bending mode of CO(2) (+)(A (2)Pi(u,12)) were assigned. The spectroscopic constants of T(e)=27 908.9+/-1.1 cm(-1) [above CO(2) (+)(X (2)Pi(g,12))], nu(1)=1126.00+/-0.36 cm(-1), chi(11)=-1.602+/-0.005 cm(-1), nu(2)(micro (2)Pi(12))=402.5+/-13.3 cm(-1), and nu(2)(kappa (2)Pi(12))=493.1+/-23.6 cm(-1) for CO(2) (+)(A (2)Pi(u,12)) are deduced from the data of the A (2)Pi(u,12)(upsilon(1)upsilon(2)0)<--X (2)Pi(g,12)(000) transitions. The observed intensity reversal between (500) (2)Pi(12) and (420)micro (2)Pi(12) can be attributed to the conformational variation of CO(2) (+)(A (2)Pi(u,12)) from linear to bent, then the conversion potential barrier is estimated to be 5209 cm(-1) above CO(2) (+)(A (2)Pi(u,12)(000)). The wavelength and level dependence of the photofragment branching ratios have been measured and the dissociation dynamics of CO(2) (+) via A (2)Pi(u,12) state is discussed.     

A study of electron scattering from benzene: excitation of the 1B1u, 3E2g, and 1E1u electronic states

We report results from measurements for differential and integral cross sections of the unresolved (1)B(1u) and (3)E(2g) electronic states and the (1)E(1u) electronic state in benzene. The energy range of this work was 10-200 eV, while the angular range of the differential cross sections was ～3°-130°. To the best of our knowledge there are no other corresponding theoretical or experimental data against which we can compare the present results. A generalized oscillator strength analysis was applied to our 100 and 200 eV differential cross section data, for both the (1)B(1u) and (1)E(1u) states, with optical oscillator strengths being derived in each case. The respective optical oscillator strengths were found to be consistent with many, but not all, of the earlier theoretical and experimental determinations. Finally, we present theoretical integral cross sections for both the (1)B(1u) and (1)E(1u) electronic states, as calculated within the BEf-scaling formalism, and compare them against relevant results from our measurements. From that comparison, an integral cross section for the optically forbidden (3)E(2g) state is also derived.