Absorption Band System Research Articles

The absorption spectrum of the H 2CSi radical

A well-resolved absorption band system, observed between 3400 and 3100 Å after a flash discharge through methylsilane, is ascribed to the free H 2CSi radical. Isotopic species D 2CSi and HDCSi are also observed. The transition is shown to be of the type 1 B 2- 1 A 1, and rotational constants are given for the lower and the upper electronic states.

Single vibronic level fluorescence of CrO 2Cl 2 excited by a tunable cw dye laser

Fluorescence of gaseous chromyl chloride has been excited using a tunable cw dye laser, and fluorescence spectra have been recorded for excitation in 20 different absorption bands. The vibrational structure of these spectra shows activity of all of the totally symmetric vibrations, but there is no evidence for activity of any non-totally symmetric vibration as previously proposed from absorption spectra. We conclude that the absorption band system near 5800 Å comprises at least two electronic transitions.

ChemInform Abstract: THE SPECTROSCOPY OF SALICYLALDEHYDES, ELECTRONIC ABSORPTION SPECTRA

Abstract Measurements of the electronic absorption spectra of vapor- and solution-phase salicylaldehydes are reported for wavelengths ≥1900 A. Three intense absorption band systems are readily assigned as A′ (ππ ∗ ) ← 1 1 A′ transitions by comparison with all-valence-electron computations. Moderate vibrational structure in the 3 1 A′ (ππ ∗ ) ← 1 1 A′ transitions of isotopic salicylaldehydes is analyzed and tentative assignments are made for the coupled vibrations.

Ultraviolet band systems of S2O

A new absorption band system of S2O has been photographed in the region 1900–2300 Å and its vibrational analysis is carried out. From this analysis the upper state vibrational frequencies are obtained as: ν2(∢S-S-O) = 270 cm−1 and ν3 (S-S) = 378 cm−1. The origin of the band system is found to be at 44 288.8 cm−1. The vibrational analysis of the 2500–3400 Å bands of S2O is also completed which yields the following data for the excited electronic state: ν1(S-O) = 1085 cm−1, ν2(∢S-S-O) = 281 cm−1 and ν3(S-S) = 422.54 cm−1, ν00 = 29 262.8 cm−1.

The spectroscopy of salicylaldehydes: Electronic absorption spectra

Measurements of the electronic absorption spectra of vapor- and solution-phase salicylaldehydes are reported for wavelengths ≥1900 Å. Three intense absorption band systems are readily assigned as A′ (ππ ∗) ← 1 1A′ transitions by comparison with all-valence-electron computations. Moderate vibrational structure in the 3 1A′ (ππ ∗) ← 1 1A′ transitions of isotopic salicylaldehydes is analyzed and tentative assignments are made for the coupled vibrations.

Study of host-guest interaction in mixed molecular crystals

The measurement of the polarization ratio (PR) of guest transitions in mixed molecular crystals has been used as a tool to study the effect of host-guest interaction. Two mixed crystal systems, namely naphthacene in trans-stilbene and anthracene in carbazole, have been studied. In the former system, the lowest energy host and guest transitions are oppositely polarized. It has been observed that in this system the ( b:a) PR value for the guest absorption system decreases progressively for the higher members of the vibronic bands as their energy difference from the host excited state decreases. This result quite convincingly demonstrates the effect of the host on a guest transition. In the latter system, the lowest energy host and guest transitions are polarized in the same direction, and in this system the ( b:a) PR value has been found to be more or less constant over the entire guest absorption band system. The results of the polarization measurement in these two mixed crystals are in conformity with the theory of host-guest interaction developed by Craig and Thirunamachandran.

Some Limitations on the Interpretation of Specular Reflection Spectra with an Application to Solid CO

A simple theoretical framework is used to calculate specular reflection spectra in the vicinity of an absorption band in a solid. The energy of the reflectivity peak is shown to differ from the energy of the absorption band even at normal incidence with thick samples and with thin film samples supported on a substrate. By considering the absorption band system of solid CO, synthetic reflection spectra of the Fourth Positive System of solid CO at 45 degrees incidence are obtained that successfully match the experimental results, including striking time-dependent changes.

Electronic spectra of 1-methoxy- and 1-ethoxynaphthalene in the vapour phase and in ethanol rigid glass at 90°K

The electronic spectra of 1-methoxy- and 1-ethoxynaphthalene in the vapour state and in alcohol rigid glass at 90°K have been studied. The vapour spectra of both the compounds show two distinct systems of absorption bands in the 2700–2900 Å and 2950–3250 Å regions. The spectra consist of strong O, O bands and several ground state and excited state vibrational modes are found to be excited. Probable nature of the perturbing vibrations and their probable correlation with corresponding modes of naphthalene have been discussed. The rigid glass spectra at 90°K are essentially similar to the vapour spectra.

Probable location of a 1E 2g state of the benzene molecule

Photoexcitation spectra of benzene in rare gas matrices show a previously unreported transition near 46000 cm −1. The observed bands are not explicable in terms of site splittings, impurity states, aggregation effects, intermediate radius states of the matrix, triplet states, excimer states, exciplex states or σ-π ∗ transitions. The vibronic spacings in these spectra could be those expected for a 1E 2g ← 1A 1g transition and on this and other evidence we argue that the ordering of origins of the first four spin allowed intravalence states of benzene is 1B 2u (38086 cm −1), 1E 2g (near 46400 cm −1), 1B 1u (48450 cm −1) and 1E 1u (55430 cm −1). Our data also show that the transition 1B 1u ← 1A 1g accounts for most of the intensity of the 210 nm absorption band system. Our ordering of the spin allowed states permits interpretation of experimental data of others, confirms certain semi-empirical and ab initio SCF MO CI calculations in which account is taken of higher excitations and illustrates the necessity of including such higher excitations. The intensity of the 1E 2g ← 1A 1g transition is at least an order of magnitude less than previously calculated indicative of the difficulty of choosing suitable wavefunctions for the 1E 2g state and of calculating “forbidden” transition probabilities.

The absorption and emission spectrum of silicon monoiodide

A new absorption band system, attributable to the silicon monoiodide molecule, SiI, has been discovered in the region 4200–4900 Å by the flash photolysis of silyl iodide, SiH 3I. The magnitude of T e and the vibrational constant ω, together with a rotational analysis of two of the bands, indicates that the system is probably the A 2 Σ +-X 2 Π 1 2 transition of SiI. An emission spectrum of SiI, first observed by Lakshminarayana and Haranath ( 1), has been photographed under higher resolution. More accurate vibrational constants have been obtained, and the electronic transition has been reinterpreted.

The extreme red absorption spectrum of Br 2, [formula omitted

The ( A 3 Π(1 u) ← X 1 Σ g + ) absorption band system of Br 2 has been fully investigated for the first time at high resolution. About 1500 lines with J″ ≦ 79 of 32 bands were measured to within 0.02 cm −1. Accurate vibrational terms and rotational constants of the A state of 79Br 79Br were found for 7 ≦ v′ ≦ 24. Two bands of 81Br 81Br were also analyzed, and confirm the numbering of the A-state vibrational levels. The derived constants reproduce the observed lines to within 0.03 cm −1.

New electronic transitions of antimony monochloride

The flash photolysis of antimony trichloride vapour under isothermal conditions yielded three systems of absorption bands in the ultraviolet region (2168–2421 Å), all being degraded to shorter wavelengths. The bands were somewhat diffuse and no isotope splittings were detectable to check the vibrational assignments. Under adiabatic conditions, only two of the band systems were observed. The known A1, A2→X(?) band systems in the visible region were not present in absorption and it is still not certain whether the lower state of any of the observed transitions of SbCl is the ground state. For all the ultra-violet systems, ω′e∼445 cm–1 and ω″e∼370 cm–1.

Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Chlorofluoromethane. The Infrared and Ultraviolet Spectra of the Free Radical ClCF

The vacuum-ultraviolet photolysis of CH2ClF and of CD2ClF isolated in an argon matrix at 14°K leads to the appearance of prominent absorptions at 742 and at 1146 cm−1 which have been assigned to the two stretching fundamentals of ClCF. A weak absorption band system between 3900 and 3400 Å, with a 376 ± 10 cm−1 average band spacing, may be tentatively attributed to ClCF, as may be an emission band system which appears between 4000 and 4900 Å when the sample is excited by 3650-Å radiation. The 379 ± 10 cm−1 average band spacing associated with this emission band system has been tentatively attributed to the bending mode of ground-state ClCF, permitting a normal coordinate analysis for this molecule. The bending and the C–Cl stretching modes are appreciably mixed. Several other groups of absorptions also have been observed in these experiments, but the species responsible for them have not been definitively identified.

Absorption edge doubling in the HD dissociation continuum

Herzberg has recently observed an absorption band system and dissociation continuum in HD analogous to the H 2 system B′ 1 Σ u + ← X 1 Σ g + . Two absorption edges, separated by 21.5 ± 0.3 cm −1, are seen at the low-frequency limit of the dissociation continuum; the intensity of absorption above the upper edge is roughly twice that between the two edges. The observed edge splitting is some-what less than the term value difference between [ D(1s) + H ∗(2s, 2p) ] and [ D ∗(2s, 2p) + H(1s) ] of 22.37 cm −1. In addition, an intense, line-like feature is observed at 14.5 cm −1 above the lower edge. Absorption edge doubling can be quantitatively described by the two-channel scattering theory developed in this paper. The electronic transition involved is assumed to be the same as that of H 2. Computations using potential curve data for the B′ 1 Σ u + state of H 2 and simple parametric forms for the exciton splitting between this state and a corresponding 1 Σ g + state reproduce not only the basic two-edge structure observed, but also predict the existence of a resonance in the photoabsorption cross section at the position observed. The general structure of edge-doubling absorption profiles and their dependence on system parameters has been investigated using analytically solvable models. It is found that a narrow interthreshold feature like that observed in HD can occur only under certain very precise “tuning” conditions, requiring a coincidence of a virtual bound level resonance of the B′ 1 Σ u + state with a similar level resonance of the 1 Σ g + state. The computations using H 2 potentials verify the sensitivity of this tuning. The narrow resonance in HD is therefore evidence that the (unknown) ( E′) 1 Σ g + potential is attractive, but assignments v′ g′ , v u ′ for the coincident level pair cannot be made without information from the discrete HD spectrum. The model studies also show that when an upper absorption edge occurs it is always located at the upper threshold, but in certain cases the lower edge may appear shifted upward from threshold and slightly broadened. Assuming this effect is the explanation of the ∼1 cm −1 edge-splitting discrepancy, we conclude that the upper edge provides the more accurate datum for the HD dissociation energy.

Absorption Spectrum of the Argon Molecule in the Vacuum-uv Region

The absorption spectrum of diatomic argon has been investigated in the 780–1080-Å region with a 6.65-m normal incidence vacuum spectrograph using the helium and argon continua as background sources. Nine discrete band systems are identified. The present analysis shows that the ground state is stable, has a dissociation energy D00 = 76.9 cm−1, and six vibration levels, υ = 0–5. A number of previous calculations of intramolecular potentials agree with these experimental results. Definite dissociation products are assigned for some of the upper states of these systems. State designations, stabilities, and transitions from the ground state are discussed by analogy with an earlier study of the xenon molecule. In some of the upper states, the existence of potential humps (for which no definitive explanation is yet known) is demonstrated by the assignment of transitions to bound molecular states above the dissociation limits and by the onset of diffuseness in band progressions. The origin of the vacuum-uv argon emission continuum and of other emission features is discussed in relation with the observed absorption band systems.

The B3Π0+-X1Σ+systems of79Br35Cl and81Br35Cl

The visible absorption band systems of 79Br35Cl and 81Br35Cl have been observed for the first time. A rotational analysis of four bands has been made, and suggests the presence of a small maximum in the potential energy curve for BrCl(3 Pi 0+), similar to those found for the 3 Pi 0+ states of ICl and IBr.

Matrix-Isolation Study of the Vacuum-Ultraviolet Photolysis of Difluorosilane. The Infrared and Ultraviolet Spectra of the Free Radical SiF2

The vacuum-ultraviolet photolysis of difluorosilane in inert matrices at cryogenic temperatures has been found to lead to the stabilization of an appreciable concentration of SiF2. A system of ultraviolet absorption bands observed in a neon matrix corresponds very well with the 0v2′0–000 bands recently reported for gas-phase SiF2 produced by another technique. In an argon matrix, the bending fundamental of SiF2 appears at 343 cm−1, and the two stretching fundamentals at 843 and at 855 cm−1. Evidence is presented supporting a previous assignment of these two fundamentals to the symmetric and antisymmetric stretching modes, respectively. Carbon monoxide matrix observations indicate that H-atom detachment plays an important role in the photolysis of difluorosilane. There is no evidence for the occurrence of processes involving F-atom detachment. It appears likely that an appreciable concentration of HSiF2 is also stabilized in the present experiments.

Induced Color Centers in CaF2Crystals Irradiated with Neutrons at Liquid Nitrogen Temperature

Effect of neutron irradiation at LNT on the color center formation in CaF 2 crystals was investigated. A peculiar absorption band system was obtained. It differs either from the usual four band spectrum or from the spectrum of the crystals neutron-irradiated at pile temperature. In addition to the bands observed in CaF 2 crystals irradiated with neutrons at pile temperature, appears a band peaking at 580 nm, which grows with increasing neutron dose. The F band could not be observed. Change of the defect state during storage and effect of temperature rise on color centers are discussed and used for the explanation of the absence of the F band in crystals irradiated at low temperatures. Optical bleaching study shows that the main band at 420 nm is composed of at least two different bands. A band at 535 nm, which is characteristic of neutron irradiation, shows a fine structure at liquid helium temperature. Brief comments are made on the center responsible for this band.

The ultraviolet absorption spectrum of the F 2 CN radical

A new system of absorption bands near 3600 Å has been observed during the flash photolysis of CF 3 NCF 2 and is ascribed to the free F 2 CN radical. The rotational analysis of the 0–0 band leads to the ground state molecular structure r CF = 1.310 Å (assumed), r CN = 1.265 ± 0.02 Å, FCF angle = 113.5 + 1°. The bands are shown to be type A bands arising from the transition 2 A 1 ← 2 B 2 , and the spectrum is compared with those of the iso-electronic molecules NO 3 and F 2 BO.

Low temperature absorption spectra of KMnO4 in KClo4

The permanganate spectrum has been remeasured at liquid hydrogen and helium temperatures using spectrographs with reasonably high dispersion. Evidence is found for four absorption band systems. The first one is assigned as 1 A 1 → 1 T 2 and displays a great deal of vibrational structure starting with the 0–0 line located at 18,072 cm−1. The second transition found at 25,000 to 30,000 cm−1 is rather featureless. The third transition with maximum at 33,000 cm−1 shows a simple progression in quanta of 750 cm−1. The fourth band is completely featureless with maximum intensity at 43,500 cm−1. These findings are compared with a simplified SCF LCAO MO calculation.