Predissociation In State Research Articles

The Spectroscopy of C2: A Cosmic Beacon.

ConspectusDicarbon, the molecule formed from two carbon atoms, is among the most abundant molecules in the universe. Said by some to exhibit a quadruple bond, it is bound by more than 6 eV and supports a large number of valence electronic states. It thus has a rich spectroscopy, with 19 one-photon band systems, four of which were discovered by the author and co-workers. Its spectrum was among the first to be described: Wollaston reported the emission spectra from blue flames in 1802.C2 is observed in a variety of astronomical objects, including stars, circumstellar shells, nebulae, comets and the interstellar medium. It is responsible for the green color of cometary comae but is not observed in the comet tail. It can be observed in absorption and emission by optical spectroscopy in the infrared, visible, and ultraviolet regions of the spectrum, and because it has no electric-dipole-allowed vibrational or rotational transitions, its spectral signature is a sensitive probe of the local environment.Before the work described in this Account, models of C2 photophysics included the thitherto-unobserved c3Σu+ state and parametrized the strength of spin-forbidden intercombination transitions. Furthermore, they did not account for photodissociation of C2, even though it was identified in the 1930s as a key process. Inspired by the observation of C2 in the Red Rectangle nebula, the author was motivated to instill rigor into C2 models and embarked on a spectroscopic and computational journey that has lasted 15 years.We were the first to identify the c3Σu+ state through the d3Πg-c3Σu+ transitions, which were to become known as the "Duck" system. This minor partner to the well-known Swan bands is a key part of astrophysical C2 models and can now be included with rigor. We identified the e3Πg-c3Σu+ system, and the c3Σu+ state is now well-studied. Meanwhile others described the singlet-triplet and triplet-quintet interactions in exquisite detail, allowing rigorous modeling of the a-X and c-X intercombination transitions.The final piece of the C2 puzzle would be understanding how long it survives before being broken into carbon atom fragments. Though predicted by Herzberg, predissociation in the e3Πg state had never been observed. To find it would require the complicated ultraviolet spectroscopy of C2 to be disentangled. In so doing, we identified the 43Πg and 33Πg states of C2, thus uncovering two new band systems. The 43Πg state allowed the first accurate determination of the ionization energy of C2. With these new band systems secure, we extracted new levels of the D1Σu+ state (Mulliken bands) and the e3Πg state (Fox-Herzberg bands) from our spectra. Upon climbing the energy ladder in the e3Πg state to v = 12, we finally identified the route to predissociation of C2 via non-adiabatic coupling to the d3Πg state. This observation provided the first laboratory evidence for why C2 is observed in the coma of a comet but not the tail.

Photodissociation spectroscopy of the Mg +–C 2H 2 π-complex

The Mg +–acetylene complex is produced by pulsed-nozzle laser vaporization and studied with mass-selected resonance-enhanced photodissociation spectroscopy. A progression of broad vibrational bands is observed near 347 nm. The broad linewidths, which preclude rotational resolution, are attributed to rapid predissociation in the excited state. The progression is assigned to the metal–acetylene stretch in the 2 B 1← X 2 A 1 band system, which produces vibrational constants of ω e′=330.5 cm −1 and D 0′=13100 cm −1. The ground-state dissociation energy is estimated as D 0″=6100 cm −1 (17.4 kcal mol −1). These properties are in good agreement with the predictions of theory.

Photodissociation spectroscopy and dynamics of MgC2H4+

The weakly bound ion–molecule complex MgC2H4+ has been studied by photodissociation spectroscopy in a reflectron time-of-flight mass spectrometer over the spectral range 218–510 nm. Mg+ is the major photofragment throughout this range, although for λ<270 nm, charge-transfer dissociation to C2H4+ is observed as a minor channel. We have identified five absorption bands of MgC2H4+. The spectral assignment is facilitated by results from ab initio calculations for the ground and low-lying excited states of MgC2H4+. Three of the bands, 1 2B2←1 2A1, 1 2B1←1 2A1, and 2 2A1←1 2A1, are based primarily in the metal-centered Mg+(3p 2P←3s 2S) atomic transition. One of the remaining bands is assigned as 2 2B2←1 2A1, a transition correlating with the a 3B1u←X 1Ag forbidden band of C2H4, with mixed charge-transfer character. The final band, 3 2A1←1 2A1, is assigned to a metal-to-ligand charge-transfer transition, enhanced by coupling with the nearby 2 2A1 state. The 1 2B2←1 2A1 band is a broad continuum, indicative of fast predissociation in the upper state. A nonadiabatic dissociation mechanism involving C=C π-bond activation by Mg+(3p) is suggested by ab initio calculations. The 1 2B1←1 2A1 band shows pronounced vibrational structure with a strong progression in the Mg+–CH4 intermolecular stretch (ν2), and weaker progressions assigned to combination bands built on the intermolecular out-of-plane wag (ν3), and a CH2–CH2 wag (ν7). The observed vibrational constants are ω2=329, x22=−2.3, ω3=439, and ω7=1024 cm−1. Measurement of the photofragment kinetic energy release determines the bond dissociation energies for the ground state [D0″(Mg+–C2H4)=0.7±0.2 eV], and for the 1 2B1 excited state, [D0′(Mg+–C2H4)=1.8±0.2 eV]. Spectroscopic constants are in good agreement with ab initio predictions.

Photofragmentation spectroscopy of MgC 2H +4

MgC 2H + 4 has been studied by photodissociation spectroscopy. We identify two distinct molecular absorption bands correlating with Mg + (3p π−3s σ) excitation in the spectral range 350–510 nm. The lowest energy band, assigned as 1 2B 2 ← 1 2A 1, is a structureless continuum indicative of a fast predissociation in the upper state. The higher energy band, assigned as 1 2B 1 ← 1 2A 1, exhibits vibrational structure, including a progression in the Mg +C 2H 4 intermolecular stretch ( ν 2), and combination bands built on the Mg +C 2H 4 intermolecular out-of-plane wag ( ν 3) and a CH 2CH 2 wag ( ν 7). The vibrational constants for these modes are ω 2 = 329 cm −1, x 22 = −2.3 cm −1, ω 3 = 439 cm −1, and ω 7 = 1024 cm −1.

New Rydberg–Rydberg transitions of the ArH and ArD molecules. I. Emission from np states of ArD

The ground electronic state of argon hydride has a repulsive potential apart from a long-range van der Waals minimum, but the Rydberg excited states have bound potentials similar to those of the ion ArH+. These states can be described approximately in terms of united-atom quantum numbers nl. We report here rotational analyses of the bands 5p→5s, 5p→6s, and 6p→5s of ArD, which help to further characterize the np Rydberg series. In ArH the bands 5p→5s and 6p→5s have broad lines because of predissociation in the lower state, and 5p→6s is difficult to analyze without further information. The present data are fitted with a Hund’s case (d) effective Hamiltonian. In previous work the 4p state was found to have a very small σ-π splitting, but this does not hold for the higher np states, and is probably due to an accidental cancellation between electrostatic and polarizability contributions. On the other hand, the spin–orbit coupling decreases monotonically with n. Features of the rotational levels are discussed in terms of the high-J limiting quantum numbers lJ=N−R and sJ=J−N, where R≡N+, in particular the effect of spin–orbit coupling on the levels with (lJ,sJ)=(−1,1/2) and (0,−1/2), which produces a tendency to Hund’s case e behavior in 4p, and a sharp avoided crossing in 6p. The corresponding avoided crossing in 5p would occur beyond the present range of observed J values.

A new electronic transition of the PCl molecule

A new transition of PCl in the vacuum UV near 130.0 nm has been observed in absorption. The identity of the carrier and the vibrational assignment of the observed bands are confirmed by the isotopic effect due to chlorine isotopomers. A predissociation in the upper state is observed from which an upper limit of the dissociation energy of the ground state of PCl is set at .

The structure, spectroscopy, and excited state predissociation dynamics of GeH2

The spectroscopy and excited state dynamics of à 1B1 germylene (GeH2) have been investigated experimentally and theoretically. Jet-cooled laser-induced fluorescence spectra of GeH2 were obtained by subjecting germane (GeH4) to an electric discharge at the exit of a pulsed nozzle. The band origins of ten vibronic transitions were determined, giving values for the upper state fundamentals of ν1=783.0 cm−1 and ν2=1798.4 cm−1. Sufficient numbers of 000 band rovibronic transitions were observed to give the ground and excited state structures as r″=1.591(7) Å, θ″=91.2(8)° and r′=1.553(12) Å, θ′=123.4(19)°. Fluorescence lifetime measurements show that the 00,0 rotational levels decay radiatively; higher J rotational states in the 00 vibronic level decay much faster, due to a heterogeneous predissociation in the excited state. High quality ab initio studies are consistent with a model in which the lower vibronic levels of the à state predissociate through the ã 3B1 state to produce Ge(3P)+H2(1Σ+g). The transition state for this process has been located and the barrier to dissociation is 15.2 kcal/mol above the à 1B1 state, so that tunneling through the barrier must occur. Above 4000 cm−1 of vibrational energy in the à state, a breaking off of fluorescence is observed as a second predissociation channel involving GeH2(à 1B1)→Ge(1D)+H2(1Σ+g) becomes accessible. This process is also found to have a barrier, in contrast to previous theoretical studies of SiH2, where the analogous dissociation was predicted to be barrierless.

Cavity ringdown laser absorption spectroscopy of the jet-cooled aluminum dimer

The 2 3Π−X 3Π electronic band system of the aluminum dimer has been measured with the CRLAS technique. While several vibronic bands were examined under high resolution, no rotational features could be discerned. Band contours imply a rapid ( τ < 10 ps) predissociation in the upper state, explaining why this band system was not observed in previous R2PI and LIF studies. The upper electronic state was determined to have a vibrational frequency of 210 ± 4 cm −1.

The dynamics of photodissociation of the gas phase N 2O·H 2O) + cluster ion

The photodissociation dynamics of the positive cluster ion of N 2O and H 2O have been examined for the wavelength range 657–458 nm (1.89–2.71 eV). The major products at all wavelengths are N 2O + H 2O, which are formed by excitation to a repulsive upper surface followed by rapid dissociation. The next most abundant products are H 2O + N 2O, which are formed predominantly by rapid dissociation from a repulsive surface. At the longest wavelength (657 nm), there is evidence for a second mechanism for production of H 2O + + N 2O, possibly involving a bound excited state. A minor N 2OH + + OH channel is observed at all wavelengths, and arises by vibrational predissociation in the ground state after photoexcitation to a bound excited state. Molecular orbital calculations indicate that at least two isomers of the cluster ion exist on the ground state potential energy surface. One isomer is best represented as N 2O + · H 2O and is a logical precursor for the N 2O +/H 2O products and the major portion of the H 2O +/N 2O products that is formed by a repulsive dissociation mechanism. The second isomer is N 2OH +·OH, probably the precursor for the N 2OH +/OH products and perhaps a minor portion of the H 2O +/N 2O products observed from excitation at 657 nm. Phase space modeling of the N 2OH + kinetic energy release distribution suggests a binding energy of about 1.2 eV for the N 2OH +·OH cluster. The relative binding energy from ab initio calculations suggests that the N 2O +/H 2O cluster is bound by about 0.5 eV. Mechanisms for formation of the observed photofragments are proposed.

A study of the predissociation in the A1Π state of BH

The predissociation in the A1Π state of BH has been studied using time-resolved spectroscopy. The radiative lifetime of the A1Π state has been estimated by the calculation of accurate ab initio wavefunctions using the CASSCF technique. The nonradiative lifetimes were calculated using a potential constructed from RKR and ab initio information and compared with the experimental results. A substantially increased value of the barrier height is shown to be necessary for a better agreement between theory and experiment.

Spectra of rare gas hydrides

Extensive emission bands of KrH and KrD have been observed in the near infrared, visible and ultraviolet regions. Most of them can readily be arranged in Rydberg series with nsσ, npσ, npπ, nd and nf upper states and a 5sσ 2Σ lower state. There is a strong resemblance of the diagram of electronic states of KrH to that of the electronic states of the united atom Rb. The present paper deals with the electronic transitions of KrH and KrD corresponding to the 2 S and 2 P states of Rb. Difficulties (not yet entirely overcome) arise with the nd and nf complexes which will be discussed in later papers of this series. The present paper deals only with the 2Σ and 2Π states arising from ns and np orbitals. The conventional molecular constants have been determined. For most electronic states only v = 0 levels have been observed but in a few cases also v = 1 and v = 2 have been observed from hot bands. No bands with v′ ≠ v″ have been found and thus no reliable vibrational frequencies are obtained. The rotational constants of KrH are very similar in value to those of KrH+ as is to be expected for Rydberg states. As for ArH the line widths of the band lines of KrH involving the lowest 2Σ state is not negligible (0·5 cm-1) indicating predissociation in this 2Σ state. This predissociation is presumably caused (as in ArH) by interaction with the repulsive ground state arising from ground state Kr + H. It is however distinctly less strong than in ArH. Indeed in KrD (unlike ArD) line broadening by predissociation is undetectable.

BaI product state distribution from the reaction Ba+CF3I

Under single-collision beam–gas scattering conditions the Ba+CF3I→Bal+CF3 reaction has been studied using laser induced fluorescence to detect the BaI X 2∑+ product. The resulting BaI C 2Π–X 2∑+ excitation spectrum has a complex appearance owing to the similarity of the upper and lower state rotational constants, causing a reversal in band shading, and owing to a predissociation in the upper state, causing a break off in the fluorescence of high vibrational levels in each spin-orbit subband. From the predissociation onset an upper bound of 78.5±0.5 kcal/mol is placed on the BaI bond energy. The vibrational population distribution is shown to be bell shaped, peaking near v″=50, and accounts for most of the available energy. The average energy appearing in BaI rotation decreases with increasing BaI vibrational excitation. The BaI rotational distribution has a width comparable to that of the vibrational distribution.

The electronic spectrum of bismuth monofluoride: A reinvestigation of the A0 +- X10 + system

A reinvestigation of the A0 +- X 10 + band system, the most electronic transitions reported for bismuth monofluoride, is presented. Approximately 65 bands were photographed in emission between 4220 and 5417 Å, of which 33 were rotationally analyzed. Inconsistencies in earlier vibrational analyses were resolved. Rotational constants for 0 ≤ ν′ ≤ 12 and 0 ≤ ν″ ≤ 18 were determined from direct least-squares fitting to the measured line positions of each band. “Merging” of the overdetermined constants was performed to obtain the best single estimate of each parameter. The resulting minimum-variance linear unbiased estimates were used to calculate RKR potential curves. Franck-Condon factors and r centroids were subsequently obtained and are presented. Evidence is given for predissociation in the upper state; perturbations involving levels above the crossing point are correlated with those previously reported in the B0 + state. The upper limit of the ground-state dissociation energy (3.14 eV) is in accord with extrapolated values as given in earlier investigations. The nature of a postulated third interacting state is also discussed.

Laser-induced predissociation of O2+(b4Σg-←a4Πu) in an ion beam

We have developed a sensitive method for studying the electronic absorption spectra of molecular ions which undergo predissociation in the excited state. The method is applied to the 4-1 band of the b 4Σg←a 4Πu transition in O2 + and rotational lines of varying width are obtained. The excited predissociating state is unambiguously assigned and refined values for both ground and excited state molecular constants are derived. Analysis of the linewidths suggests that predissociation of the b 4Σg - state probably occurs through coupling with a 4Σg + state.

Spectra of the d1Σ+–c1Π and d1Σ+–b1Σ+ systems and dissociation energy of NH and ND

Spectra of the d1Σ+–c1Π system of NH and ND and the d1Σ+–b1Σ+ system of NH have been observed which substantially extend previous data. The following equilibrium constants have been derived (in units of cm−1):[Formula: see text]Observation of predissociation in the c1Π state yields the following upper limits on the dissociation energy: D00 (NH) ≤ 3.47 eV and D00 (ND) ≤ 3.54 eV.

The ultraviolet spectrum of AsO

The emission spectrum of As 16O and As 18O was excited in low pressure vapors and photographed with dispersions between 2.5 and 0.1 Å/mm. Vibrational isotope analysis confirmed the earlier assignment of bands in the systems A and B, but lead to a new assignment of bands belonging to the C- X and D- X system. A rotational analysis of the B- X system indicated that the transition is 2Σ +- 2Π (a) with B′ = 0.525 cm −1 and B″ = 0.492 cm −1. A strong predissociation in state B at v = 0 and N = 20 (As 18O: N = 23) indicates that the dissociation energy of AsO is ≧ 40 100 cm −1 (4.98 eV). From partial analysis of fine structure it was concluded that the character of states A and D is A 2Σ and D 2Δ with a B A ∼ 0.46 and B D ∼ 0.43 cm −1.

ON THE DISSOCIATION OF PbF AND BiF MOLECULES

True potential energy curves have been calculated for the A and X states of BiF and PbF molecules using the Rydberg–Klein–Rees (R.K.R.) method as modified by Vanderslice et al. It has already been shown that by fitting an empirical potential function to the actual potential (R.K.R.) curve of a state we can obtain an idea of the correct dissociation energy of the molecule in that particular state. The three-parameter Lippincott function has been used for this purpose. The resulting dissociation energies for the ground states of PbF and BiF are (2.4 ± 0.2) eV and (2.60 ± 0.2) eV respectively. In PbF a large number of band systems are known, two of which show predissociation in the upper excited state. It has been found possible to account for both of these predissociations in PbF as being due to the A state of the molecule.

EMISSION BAND SPECTRA OF NITROGEN THE LYMAN-BIRGE-HOPFIELD SYSTEM

The near ultraviolet part of the emission spectrum of nitrogen has been photographed under high resolution. Thirteen bands of the [Formula: see text] system (Lyman–Birge–Hopfield) have been analyzed and new vibrational and rotational constants obtained. Combining the observed data with those obtained by Stoicheff from the Raman spectrum of nitrogen, refined equilibrium constants for the ground state were obtained. The predissociation in the α1Πg state was observed.

PREDISSOCIATIONS OF THE C12O AND C13O MOLECULES

The Ångström bands of C12O and C13O have been photographed with a high dispersion spectrograph and the predissociations previously reported in these bands have been re-examined. The predissociations provide good evidence that C12O has a dissociation limit 89595 ± 30 cm.−1 above the ν = 0 level of the ground state. The fourth positive (A1Π − X1Σ) bands have been re-examined and it has been shown that, contrary to earlier reports, there is no evidence for predissociation in the A1Π state.

PREDISSOCIATION AND DISSOCIATION OF THE N2 MOLECULE

The predissociation in the upper state of the Lyman–Birge–Hopfield bands of nitrogen has been investigated under high resolution. A clear breaking-off of the fine structure has been found above ν′ = 6, J′ = 13 both in the P and the Q branches. The predissociation limit is at 78,843 ± 50 cm.−1 which is smaller than the well known dissociation limit 97,944 cm.−1 by very nearly the excitation energy of the 2D state of the N atom. This observation eliminates the intermediate value, 8.565 ev., for the dissociation energy of the N2 molecule. A definite decision between the two remaining values, 9.756 and 7.373 ev., cannot yet be made. Acceptance of the former would involve the assumptions that the predissociation in the a1Πg state is caused by the [Formula: see text] state resulting from normal atoms and that this [Formula: see text] state is not purely repulsive. The elimination of the value 8.565 ev. for D(N2) removes the value 7.50 ev. for D(CN) which has been fairly widely accepted.