Rydberg states of carbon dioxide and carbon disulfide

The electronic absorption spectra of carbon dioxide and carbon disulfide have been reexamined. Model potential calculations have been used to calculate the energies of excited states in Rydberg approximation, and (npσ) and (npπ) Rydberg series have been assigned. For both molecules, the lowest excited ¹Π_g and ¹Π_u states are identified as Rydberg states. The lowest ${}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state is mainly Rydberg for CO₂, but contains some valence character for CS₂, There is no evidence for transitions to additional valence states of these symmetries.It is shown that $\text{LCAO}\text{MO}$ predictions about excited states can be misleading because of near-linear dependencies which arise in multicenter expansions. A consideration of the united atom orbitals for CO₂ and CS₂ predicts that there should be only the number of low-energy excited states which are found from the spectral analysis.     

Visible and near infrared emission spectra of PCl excited in the reaction of Ar(3P2, 0) with PCl3: Vibrational analysis of PCl A3Π → X3Σ− and b1Σ+ → X3Σ−

Two band systems of PCl, A³Π_r → X³Σ^? and b¹Σ⁺ → X³Σ^?, have been observed from the reaction of $\text{Ar}\text{(}{}^3\text{P}{}_{\mathrm{2,\; 0}}\text{)}$ with PCl₃ at pressures of 1–2 Torr. Seventy-eight bands of P³⁵Cl and 31 bands of P³⁷Cl in the region 4000–6000 Å have been assigned to the A³Π_r → X³Σ^? system and include levels with v′ = 0, 1, 2 and v″ = 3–18. The ground state numbering was obtained from a study of the vibrational isotope effect. The (0,0) sequence of the b¹Σ⁺ → X³Σ^? system occurs near 8200 Å and has been observed up to v′ = 10. Vibrational constants for all three states are derived from least-squares fits of the measured bandhead wavenumbers. The A ← X absorption system of PCl reported by Basco and Yee in flash photolysis of PCl₃ was not observed, and is probably due to absorption to a Rydberg state of PCl.     

Two emission systems of BS

Two new systems of emission bands near 2100 and 3100 Å have been produced by a microwave discharge in B₂S₃ vapor. From the known X²Σ⁺ and A²Π_i states of BS, these systems have been assigned as E²Σ⁺ → X²Σ⁺ and E²Σ⁺ → A²Π_i. Constants in cm^?1 for the new state are

$\text{E}{}^2\text{∑}{}^{\mathrm{\pm }}\text{: T}{}_{\mathrm{e}}\text{= 47 929.3, B}{}_{\mathrm{e}}\text{= 0.671 (λ}{}_{\mathrm{e}}\text{= 1.752 A), α}{}_{\mathrm{e}}\text{= 0.008}$

,     

Energy levels of low lying triplet states of N2 from infrared emission studies

A theoretical model used to describe the $\text{B′}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{?}}$ and B³Π_g states of N₂ is presented. Using recently acquired high resolution spectra of the $\text{B′}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{?}}\text{→ B}{}^3\text{Π}{}_{\mathrm{g}}$ (0-0) band, rotational energy levels of the v = 0 vibrational levels of these two states are generated with this model. These levels are in excellent agreement with those obtained using a combination differences technique. The precision of the model generated levels is 0.01 cm^?1. The previously unpublished rotational levels of Dieke and Heath for the $\text{A}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$, B³Π_g and C³Π_u states are referenced to the $\text{N}{}_2\text{X}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ (v = 0, J = 0) ground level and tabulated here. Estimates of the precision of their work are made.     

Molecular Rydberg transitions: Cyanogen chloride

The electronic absorption spectrum of cyanogen chloride has been investigated in the range 2200-1250 Å. The first s-Rydberg transitions, $\text{X}\text{?}{}^1\text{Σ}{}^{\mathrm{+}}\text{→}{}^3\text{Π}{}_1$ and $\text{X}\text{?}{}^1\text{Σ}{}^{\mathrm{+}}\text{→}{}^1\text{Π}{}_1$ have been assigned, and analyzed to yield exchange and spin-orbit coupling parameters. The relative intensities of these two transitions have been shown to accord with an intermediate coupling situation. The $\text{π → π}{}^1$ intravalence excitations, leading to ^1.3(Σ^?, Δ and Σ⁺) states, have been discussed. It has been shown that one or both of the ¹Σ^? and ¹Δ states have bent geometries and that the ¹Σ⁺ state is located (tentatively) at 79 755 cm^?1. Two $\text{σ → π}{}^1\text{π → σ}{}^1$ states have been assigned, one at 56 340 cm^?1, the other at 74 450 cm^?1. The latter assignment is tentative, being largely based on observed vibronic interferences between the $\text{X}\text{?}{}^1\text{Σ}{}^{\mathrm{+}}\text{→}{}^1\text{Π}{}_1$ transition and the 74 450 cm^?1 transition. A considerable amount of vibrational oscillator strength and quantum defect data is presented.     

Absorption spectrum of CO in the Hopfield helium continuum region: Rydberg bands converging to the X2Σ+ state of CO+ in the region 960–1080 Å

The high resolution absorption spectrum of CO has been reinvestigated in the Hopfield helium continuum region, particularly from 960 to 1080 Å. The Rydberg state 3pπE¹Π was extended to v = 2, and other Rydberg states, $\text{3dσ}{}^3\text{Σ}{}^{\mathrm{+}}$ and F¹Σ⁺, $\text{3dπ}{}^1\text{Π}$, and $\text{4sσ}{}^3\text{Σ}{}^{\mathrm{+}}$ and ¹Σ⁺, which are converging to the X²Σ⁺ ground state of CO⁺, have been identified. The rotational structures of only five bands among the observed ten Rydberg bands have been analyzed.     

The B2Σ+ → A2Πi system of silicon nitride,SiN

The 440-nm violet-degraded ${}^2\text{Σ →}{}^2\text{Π}$ bands of SiN, which were previously assigned to a $\text{“K” → A}$ system, have been reanalyzed. These bands are shown to be Δv = 0, ±1 sequence bands of the B²Σ⁺ → A²Π system of SiN. The first reliable value of T_e(A²Π) = 994.4(1) cm^?1 has been obtained, and this determines the location of the D²Π and L²Π states with respect to the ground state. The B²Σ⁺, v = 7 and D²Π, v = 3 levels are shown to be mutually perturbing. A detailed study has been made of the perturbed X²Σ⁺, v = 8 level. The 6–8 band of the B → X system has been photographed at high resolution. A deperturbation of this band confirms T_e(A²Π), and provides the first experimental verification of the inverted nature of the A state.     

Predissociation and Λ-doubling in the even-parity Rydberg states of the nitrogen molecule

Predissociations in the y¹Π_g and $\text{x}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{?}}$ Rydberg states of N₂ (configurations $\text{1π}{}_{\mathrm{u}}{}^{\mathrm{?1}}\text{4pσ}$ and $\text{1π}{}_{\mathrm{u}}{}^{\mathrm{?1}}\text{3pπ}$, respectively) and their likely causes, are discussed. Peaking of rotational intensity at unusually low J values, without sharp breaking off, is interpreted as due to case c^? or case cⁱ predissociation. Λ doubling in the y state, attributed to interactions with the $\text{x}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{?}}$ state and with another, ¹Σ⁺, state of the same electron configuration as x, is analyzed. From this analysis the location of the (unobserved) ${}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ state, here labeled x′, is obtained. It is concluded that the predissociation in the Π⁺ levels of the y state is an indirect one mediated by the interaction with x′ coupled with predissociation of x′ by a ${}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{?}}$ state dissociating to ${}^4\text{S +}{}^2\text{P}$ atoms: combined, however, with perturbation of the y state by the k¹Π_g Rydberg state (configuration $\text{3σ}{}_{\mathrm{g}}{}^{\mathrm{?1}}\text{4dπ}$), whose Π⁺ levels are completely predissociated.     

Spectrum and structure of the He2 molecule: Characterization of the singlet and triplet states associated with the UAO's 4s, 4dσ, 4dπ, and 4dδ

The emission spectrum of the He₂ molecule has been rephotographed in the ～4000–～5700 Å region and the $\text{4d(}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{,}{}^3\text{Π}{}_{\mathrm{u}}\text{,}{}^3\text{Δ}{}_{\mathrm{u}}\text{) → 2pπ}{}^3\text{Π}{}_{\mathrm{g}}$, $\text{4d(}{}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{,}{}^1\text{Π}{}_{\mathrm{u}}\text{,}{}^1\text{Δ}{}_{\mathrm{u}}\text{) → 2pπ}{}^1\text{Π}{}_{\mathrm{g}}$, $\text{4s}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{→ 2pπ}{}^3\text{Π}{}_{\mathrm{g}}$ and $\text{4s}{}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{→ 2pπ}{}^1\text{Π}{}_{\mathrm{g}}$ transitions analyzed. The 4dδj³Δ_u, 4dπj³Π_u, $\text{4dσj}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ and $\text{4sh}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ states have been characterized through v = 2 and the 4dδJ¹Δ_u, 4dπJ¹Π_u, $\text{4dσJ}{}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$, and $\text{4sH}{}^1\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ states for v = 0. The term levels for these perturbed and l-uncoupled states have been confirmed (a) by analyses of bands with common levels from Δv = 0, ±1 sequences and (b) by analyses of the transitions between the above states from 4d and 4s and the $\text{c}{}^3\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ and $\text{C}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ states associated with 3pσ. Molecular constants are reported which have been partially corrected for the effects of l-uncoupling and the homogeneous perturbations between the state pairs J, H and j, h.     

Calculated potential surfaces for the description of the emission spectrum of the C2H radical

Ab initio MRD-CI calculations are reported for the CC stretch and CCH bending potential surfaces for the lowest-lying electronic states of the ethynyl radical. It is found that the energies of the fully relaxed $\text{π → π}{}^1$ states of this system are as much as 2.3 eV below the corresponding values at the ²Σ⁺ ground state (linear) equilibrium structure. The fact that the energy minima for the C₂H excited states occur for widely different internuclear arrangements than that of the ground state is shown to be very consistent with the experimental observation that the electronic spectrum of C₂H has a much different appearance in emission than it does in absorption studies. On the basis of both calculated energy differences and oscillator strength results it is found that the most likely assignment for the broad emission feature in the 4000–6000 Å region of the C₂H spectrum is caused by vertical excitation from the lowest vibrational levels of the 3²A′ (and also 2²A″) upper states to the 2²A′, ²A′ and ²A″ species which correlate with X²Σ⁺ and A²Π for linear nuclear arrangements; the possible mechanisms for production of 3²A′ and 2²A″ upon photolysis of acetylene are also discussed. The possibility that spinforbidden transitions are involved at longer wavelengths in this spectral region is also considered. Finally it is pointed out that a potential crossing of the linear $\text{X}\text{?}{}^2\text{Σ}{}^{\mathrm{+}}$ and A²Π species calculated to occur in the neighborhood of R_CC = 2.6 bohr, which becomes sharply avoided upon molecular bending, would be expected to lead to the type of irregular vibronic structure in the 3600 cm^?1 region of the C₂H IR spectrum recently observed by Jacox.     

The electronic spectrum of gaseous SnS

Observations of the spectrum of SnS excited in chemiluminescence have led to the characterization of two low-lying excited states of SnS, $\text{a}\text{Ω}\text{1(}{}^3\text{Σ}{}^{\mathrm{+}}\text{)}$, with T_e = 18 143.9 cm^?1, and $\text{A0}{}^{\mathrm{+}}\text{(}{}^3\text{Π}\text{)}$, with T_e = 22 021.3 cm^?1. Extended rotational analyses of the perturbed bands observed in the absorption spectrum enable assignments to be suggested for the components $\text{Ω}\text{0}{}^{\mathrm{+}}$ and 1 of ³Σ^? and $\text{Ω}\text{1}$ of ³Π.     

Rydberg states of SiBr

New uv absorption spectra have been observed for SiBr. Five Rydberg states are identified to the states $\text{(4sσ)}{}^2\text{Σ}{}^{\mathrm{+}}$, $\text{(5sσ)}{}^2\text{Σ}{}^{\mathrm{+}}$, $\text{(4pπ)}{}^2\text{Π}$, $\text{(3dπ)}{}^2\text{Π}$, and $\text{(3dδ)}{}^2\text{Δ}$ by comparison with SiF and SiCl. The ionization potentials of SiCl and SiBr have been determined for the first time, and were 6.82 and 6.67 eV, respectively.     

Reexamination of the vacuum ultraviolet emission spectrum of CO in the 950–1200 Å region

New spectrograms of CO below 1200 Å reveal that emission bands terminating on the high energy $\text{E}{}_0{}^1\text{Σ}{}^{\mathrm{+}}$ and ¹Π states plus most of the previously reported unidentified emission bands of CO actually originate from molecular nitrogen. Four new emission band systems in CO, tentatively identified as V¹Π-X¹Σ⁺, W¹Π-X¹Σ⁺, Y¹Σ⁺-X¹Σ⁺, and Z¹Σ⁺-X¹Σ⁺, have been observed. The corresponding T_v0′s are 98917, 102804, 99963, and 105724 cm^?1, respectively.     

The emission spectra of H35Cl+, H37Cl+, D35Cl+, and D37Cl+ in the region 2700–4100 Å

The $\text{A}{}^2\text{Σ}{}^{\mathrm{+}}\text{-X}{}^2\text{Π}$ emission spectrum of HCl⁺ has been measured and analyzed for four isotopic combinations. These analyses extend previous work and provide rotational constants for the v = 0–2 levels of the ground state and for the v = 0–9 levels of the excited state. RKR potentials have been determined for both states, although the upper state could not be fitted precisely to such a model. Calculated relative intensities based on these potentials demonstrated that the electronic transition moment must change rapidly with lower state vibrational quantum number. Although considerable caution should be exercised in applying the concept of equilibrium constants to the $\text{A}{}^2\text{Σ}{}^{\mathrm{+}}$ state, the following are the best estimates of these constants (in cm^?1) for the $\text{X}{}^2\text{Π}$ state of H³⁵Cl⁺: B_e = 9.9406, ω_e = 2673.7, A_e = ? 643.7, and $\text{r}{}_{\mathrm{e}}\text{= 1.315}\text{A}\text{?}$. For the $\text{A}{}^2\text{Σ}{}^{\mathrm{+}}$ state of H³⁵Cl: T_e = 28 628.08, B_e ～ 7.505, ω_e ～ 1606.5, and $\text{r}{}_{\mathrm{e}}\text{= 1.514}\text{A}\text{?}$.     

Lifetime of electronic states of CO2+

The radiative lifetimes of the $\text{A}\text{?}{}^2\text{Π}{}_{\mathrm{u}}$ and $\text{B}\text{?}{}^2\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ states of CO₂⁺ were measured by means of the delayed coincidence method. Excitation was performed by a pulsed electron beam incident on CO₂. The results of these measurements are 115 ± 5 nsec for the $\text{A}\text{?}{}^2\text{Π}{}_{\mathrm{u}}$ state and 126 ± 3 nsec for the $\text{B}\text{?}{}^2\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}$ state.     

Model calculation of the electronic structure and spectroscopy of Hg2

Energy curves and transition moments of the excited valence states of Hg₂ were obtained in a model calculation based on calculated Mg₂ energy levels and the assumption that the asymptotic spin-orbit matrix elements for the Hg atom are applicable to the molecular states. The spin-orbit and orbital-rotational interaction of the excited states of Hg₂ is analyzed in both a Hund's case (c) and (a) representation. The intermediate (a) → (c) transition moments are obtained as a function of the internuclear distance. The effect of the orbital-rotational interaction which introduces Hund's case (b) and (e) couplings is found to be small for transitions among excited states under the conditions normally encountered for populating excimer states.Using the energy level positions and transition moments, the observed spectra and predicted spectra are compared for both radiative transitions including the ground state and among the excited states. The lifetime of the $\text{1}{}_{\mathrm{u}}\text{(}{}^3\text{Σ}{}_{\mathrm{u}}{}^{\mathrm{+}}\text{)}$ excimer state is calculated to be 1.4 μsec with the 335 nm band assigned to the $\text{1}{}_{\mathrm{u}}\text{→ X}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}$ transition. The 485 nm bands cannot be assigned to any Hg₂ transitions. Strong bound-continuum absorptions are predicted for the 485 nm bands. On the other hand, the 335 nm emission is predicted to be absorbed by bound-bound transitions only.     

Rydberg states with a 2Π core: The b3Πi and C1Π states of DCl

The rotational structure in the lowest Rydberg complex of hydrogen chloride, [X²Π]4sσ, was reinvestigated. The study is limited to the spectrum of D³⁵Cl, the HCl bands being too diffuse for a detailed analysis of second-order effects. The Λ-type doubling in both component states, b³Π_i and C¹Π, is small since it arises from the uncoupling of the core rather than Rydberg orbital angular momentum. It can be interpreted in terms of pure precession relations that are known to exist between the ground and first excited states of DCl⁺. By contrast, the spin-orbit interactions, also originating in the core, are strong. In addition to distorting the triplet splitting in b³Π, they lead to an avoided crossing between the nearly coinciding levels b³Π₀ (v = 1) and C¹Π₁ (v = 0). They are also responsible for anomalies in the b³Π₀ ← X¹Σ⁺R-branch intensities of DCl as well as of HBr, DBr, and HI. From the J values at the observed R-branch minima we have estimated the ratio $\text{μ}{}_{\mathrm{\parallel }}\text{μ}{}_{\mathrm{\perp }}$ of the transition moments associated with the excitation of a 3pσ or 3pπ core electron to the 4sσ Rydberg orbital of DCl and, correspondingly, of the other hydrogen halides.     

Revised molecular constants,RKR potential,and long-range analysis for the B3Π(0u+) state of Cl2, and rotationally dependent Franck-Condon factors for Cl2 (BX1Σg+)

Literature data for the line frequencies of the B³Π(0_u⁺) ← X¹Σ_g⁺ transition of Cl₂ are fitted directly by least squares to obtain new molecular constants. The constants from individual bands are merged to obtain single-valued estimates of the rotational constants for each vibrational level of the B state. The results are combined with recent data from the B → X system in emission to obtain new RKR turning points for the B and X states, and Franck-Condon factors for the B-X system. The new constants are also used to provide revised long-range parameters for Cl₂(B) which differ from those of earlier work. In particular, the coefficient C₅ of the leading term in the inverse-power long-range potential is now found to be $\text{C}{}_5\text{= 1.16(2) × 10}{}^5\text{A}\text{?}{}^5\text{cm}{}^{\mathrm{?1}}$. Theoretical results for the variation of centrifugal distortion parameters for levels near dissociation are tested for D_v and H_v, and an extrapolation based on this behavior is used to facilitate determination of reliable B_v and G(v) values for the highest observed B-state levels.     

Diode laser spectroscopy of the CCl radical

Vibration-rotation transitions of the fundamental band have been observed for both C³⁵Cl and C³⁷Cl in the ${}^2\text{Π}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and ${}^2\text{Π}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ states by using an infrared diode laser spectrometer with Zeeman modulation. A few lines of the “hot” band (v = 2 ← 1) have also been recorded for C³⁵Cl. From an analysis of the observed spectra improved values were obtained for the vibrational harmonic frequency and anharmonicity constant, rotational constants, and Λ-doubling parameters. It was found necessary to take into account centrifugal distortion effects on the spin-orbit coupling constant A in the analysis, which gave $\text{(}\text{dA}\text{dr}\text{)}{}_{\mathrm{<mtext>e</mtext>}}\text{r}{}_{\mathrm{<mtext>e</mtext>}}$ to be ?176 ± 38 or ?125 ± 38 cm^?1, depending upon whether ²Σ^? or ²Σ⁺ states contribute more to the Λ-type doubling. The equilibrium internuclear distance r_e was calculated from the derived rotational constants to be 1.64506 ± 0.00016 Å.     

The rotational spectrum of the X 2Σ+ state of the SrF radical using laser microwave optical double resonance

The pure rotational spectrum of the $\text{X}{}^2\text{Σ}{}^{\mathrm{+}}$ state of the gaseous SrF radical has been measured using microwave optical double resonance (MODR) techniques. The analysis fully confirms the recent dye laser excitation spectrum and rotational assignment of the $\text{B}{}^2\text{Σ}{}^{\mathrm{+}}\text{-X}{}^2\text{Σ}{}^{\mathrm{+}}$ system. Transitions were measured in both the v″ = 0 and v″ = 1 states to give values of B_e″ = 0.250533 cm^?1, α_e″ = 1.546 × 10^?3 cm^?1 and γ″ (spin-rotation) = 2.49 × 10^?3 cm^?1. General qualitative features of MODR in ²Σ⁺ states are treated and suggested improvements for obtaining experimental hyperfine constants are discussed. The more precise ground state constants are merged with the B-X optical analysis to obtain a more accurate set of constants for both states.