Accidental Resonances Research Articles

Astrophysical laser operating in the O i 8446-Å line in the Weigelt blobs of η Carinae

Within the framework of a simple model of photophysical processes in the Weigelt blobs in the vicinity of the luminous blue variable (LBV) star eta Carinae, we explain the presence of the fluorescent 8446-angstrom and forbidden [O I] 6300-angstrom lines as well as the absence of the allowed O I 7774-angstrom line in spectra recorded with the Hubble Space Telescope (HST)/STIS instrument (Gull et al.). From atomic data and estimated stellar parameters we demonstrate that there is a population inversion and stimulated emission in the 3p P-3-3s S-3 transition lambda 8446 due to photoexcitation by accidental resonance (PAR) by H Ly beta radiation.

Identification of enhanced absorption by 16O 3 lines around [formula omitted] in high-resolution FTIR solar spectra

A set of previously unpredicted 16O 3 strong absorption lines in the 5 μ m region are identified for the first time in balloon-borne high-resolution ( 0.003 cm - 1 ) solar spectra. These spectral features are consistent with recent ozone laboratory spectra, and are interpreted by accidental resonance effects. Some of the lines are also observed in high-resolution ground-based spectra.

Femtosecond pump–probe photodissociation of OClO: direct observation of the 2A 1 state and the role of accidental multiphoton resonance

Previous pump–probe experiments on the OClO molecule are analyzed and a consistent interpretation of the experimental observations is proposed. We demonstrate that a direct observation of the very weak 2A 1 state of OClO is possible with a proper femtosecond pulse, and suggest that it is necessary to introduce an accidental resonance of the probe pulse with the highly excited D or E states for understanding the observed distinct photodissociation dynamics of the 2A 2 state using pump pulses of slightly different wavelengths. A transient abnormal peak, observed in the OClO + ion signal after a stronger excitation, is also convincingly interpreted.

Ground state spectrum of methylcyanide

The rotational spectrum of methylcyanide (acetonitrile) in the ground vibrational state was measured in the spectral region from 91 to 810 GHz using the Cologne and Tsukuba spectrometers operated in the Doppler-limited and sub-Doppler saturation layouts. The resolution of the saturation Lamb-dip measurements is estimated to be about 1 kHz at the best of circumstances and the measuring accuracy of 10–60 kHz depending very sensitively on the quality of the spectrum. In the cases of rotational transitions with the low quantum number J ( J<18) and with a low difference of the rotational quantum numbers J− K, the resolved or partly resolved hyperfine structures of the rotational transitions were observed. Together with the most accurate data from the literature, the newly measured experimental data were analyzed using the traditional polynomial energy formula as well as the Padè approximant for the effective rotational Hamiltonian. The resulting rotational, centrifugal distortion, and hyperfine structure spectroscopic constants were obtained with a significantly higher accuracy than the ones listed in the literature. In addition, an anomalous accidental resonance was detected between the K=14 ground state levels and the K=12, + l levels in the excited v 8=1 vibrational state.

Article

The band complex near 107 cm −1 consisting of the fundamental ν 9 of dicyanoacetylene (NC–CC–CN) and its associated hot bands has been analysed from the high resolution infrared spectrum. It turned out that two sets of bands from levels ( v 9 =0–8) and ( v 7 =1; v 9 =0–5) were observable. Including cases where separate e - and f -transitions have been observed, the total number of identified series exceeds 90. The calculations were made with a global rovibrational analysis program adapted for linear sixatomic molecules. We have developed general procedures for calculation of the relative intensities of all allowed transitions between two vibrational polyads in the electronic ground state of a linear molecule, taking into account the wavefunction mixing due to essential or accidental resonances. This supports the assignment procedure in the present case and allows us to understand the dramatic effects on the intensities of different subbands. A good agreement between calculated and observed intensities is obtained with one single intensity parameter for the whole set of subbands.

Analysis of the ν9 band complex of dicyanoacetylene and application of a theory of relative intensities to all subbands

The band complex near 107 cm −1 consisting of the fundamental ν 9 of dicyanoacetylene (NC–CC–CN) and its associated hot bands has been analysed from the high resolution infrared spectrum. It turned out that two sets of bands from levels ( v 9=0–8) and ( v 7=1; v 9=0–5) were observable. Including cases where separate e- and f-transitions have been observed, the total number of identified series exceeds 90. The calculations were made with a global rovibrational analysis program adapted for linear sixatomic molecules. We have developed general procedures for calculation of the relative intensities of all allowed transitions between two vibrational polyads in the electronic ground state of a linear molecule, taking into account the wavefunction mixing due to essential or accidental resonances. This supports the assignment procedure in the present case and allows us to understand the dramatic effects on the intensities of different subbands. A good agreement between calculated and observed intensities is obtained with one single intensity parameter for the whole set of subbands.

Astrophysical Lasers with Radiation Pumping by Accidental Resonance

ABSTRACTIn this paper we consider a general problem of astrophysical lasers operating with optical pumping by accidental resonance (PAR) with strong spontaneous emission lines of other elements. The origin of population inversion and the coefficient of amplification due to the optical excitation of atoms (ions) by a four‐level scheme with PAR are analyzed. We elucidate conclusively the question of the manifestation of astrophysical lasers in the optical region of the spectrum, which in principle differs from that of astrophysical masers in the microwave range.

An optical–optical double-resonance study of the Rydberg states of O2. II. The np and nf (ungerade) states excited via single-rotational levels of the b 1Σ0g+ valence state

The np (n=3–10) and nf (n=4–9) Rydberg states of O2 converging on O2+ X 2Π1/2,g and X 2Π3/2g have been studied between 75 000 and 99 900 cm−1 using optical–optical double resonance with multiphoton ionization. Three-photon excitation from single rotational levels of the initially excited b 1Σ0g+ valence state was used to access these states. The nf states show a strong tendency towards (Ω,ω) coupling for all values of n, whereas the np states appear to be best described by (Λ,S) coupling for n⩽8. The intensities of some of the 5f bands are anomalously high due to accidental resonances with the 3s d 1Π1g Rydberg state at the two-photon level.

Rotational analysis of the origin and the inversion bands of the S1←S spectrum of acetaldehyde

Fully rotationally resolved spectra of the two lowest-frequency bands (the origin (000) and the inversion (1401) band) of the S1(nπ*)←S0 transition of jet-cooled (Trot≈6 K) acetaldehyde, CH3CHO, have been recorded with a resolution of ≈0.01 cm−1 using a pulsed dye amplified continuous wave (cw) laser. In modeling the spectra a nonperturbative solution of the rotational–torsional coupling problem was used, but the torsion–inversion and the rotation–inversion couplings were neglected. All the lines of the origin band were reproduced with a model using the same rotational–torsional Hamiltonian for the ground- and excited electronic state. The inversion band could not be described with this model, since the ordering of the torsional levels in the excited state is reversed. The measured spectrum was reproduced by using a rigid asymmetric rotor Hamiltonian for the two torsional levels in the excited state. Some rotational levels of 00 and 141 states were found to be shifted from their predicted energy values. These shifts were explained by an accidental resonance between the excited singlet level and some higher rovibronic triplet level. The relative intensities of the ab-type and c-type torsional subbands and the Herzberg–Teller–induced transition dipole moment direction, characterizing the ab-type subband, were determined by an intensity fit.

Properties of random state manifolds with applications to intramolecular vibrational redistribution

We investigate spectral properties of random manifolds, which consist of a bright state coupled to mutually uncoupled dark states for an equal but otherwise arbitrary distribution of the couplings. Both Poisson and Wigner distributions of the energy spacings of the dark states are taken into account. The Poisson spacing model is solved exactly. The average spectrum is Lorentzian. The average dilution factor comes out to be a function alone of the mean coupling strength normalized to the mean neighbor spacing of the dark states. A simple expression for the explored fraction of the available phase space is obtained. Numerical studies indicate that the normalized coupling even controls the whole distribution of the dilution factor for the Poisson model. For weak mean coupling strength a secondary peak occurs in this distribution for both the Poisson and the Wigner model. A perturbational analysis shows that this peak leads back to accidental resonances of the bright state with single dark states. A simple tier model is suggested for treating the intermixing of vibrational dark basis states in molecules. Results are compared with experimental data.

Influence of Vibrational Resonances and Coriolis Coupling on Dissociation Rates in the Near-threshold Unimolecular Fragmentation of HOCl

The unimolecular dissociation of HOCl in the ground electronic state near the HO+Cl fragmentation threshold is investigated employing an ab initio potential energy surface. The dynamics calculations are performed by using an imaginary absorbing potential and filter diagonalization. The computational study is mainly focused on the influence of accidental or systematic vibrational resonances on the total angular momentum dependence of dissociation rates and the importance of Coriolis interaction.

New Measurements of the Solar Limb Emission in the O [CSC]i[/CSC] 777.4 and 844.6 Nanometer Lines

Slit spectra of the solar limb emission in the 777.4 and 844.6 nm O I lines are calibrated against the solar disk center brightness. Data from CCD spectrograph instruments at two different observatories show limb emission with identical line intensities and measure a value of R≡I(844.6 nm)/I(777.4 nm)=0.20±0.02, very different from the ratios computed from early photographic measurements of these lines. Implied by this new value of R4 is that O I lines in the solar chromosphere are formed in regions with electron densities log(Ne) > 11.5 and that collisional excitation is more important than the photoexcitation by accidental resonance mechanism.

On the spin-rotation contribution to nuclear spin conversion in C3v-symmetry molecules. Application to CH3F

The symmetrized contribution of E-type spin-rotation interaction to conversion between spin modifications of E- and A1-types in molecules with C3v-symmetry is considered. Using the high-J descending of collisional broadening for accidental rotational resonances between these spin modifications, it was possible to co-ordinate the theoretical description of the conversion with (updated) experimental data for two carbon-substituted isotopes of fluoromethane. As a result, both E-type spin-rotation constants are obtained. They are roughly one and a half times more than the corresponding constants for (deutero)methane.

FT-IR Study of the Perpendicular Bands of 1,3,5-Triazine I:* The v7 and v9 Bands of 12C3 14N3H3 , 13C3 14N3H3, 12C3 15N3H3 and the Difference Band v9-v14 of 12C3 14N3H3

Abstract The analysis of the high resolution FT-IR spectra of the perpendicular bands v7 (E') at about 1550 cm -1 and V9 (E') at about 1170 cm -1 of the isotopomers 12C3 14N3H3 , 13C3 14N3H3 , and l2C3 I5N3H3 is given. Both bands proved to be free from accidental resonances. The molecular constants of the state v7 = 1 and v9 = 1 of the isotopomers under consideration are listed. The weak difference band v9-vl4 (E" → E') of 12C3 14N3H3 was recorded and analyzed, using the molecular constants of v9 = 1 [this work] and V14 = 1 [of 1995], This analysis proves the quality of the molecular constants of the fundamental v14 which is IR-inactive.

Infrared Spectrum of Ozone in the 4600 and 5300 cm−1Regions: High Order Accidental Resonances through the Analysis of ν1+ 2ν2+ 3ν3− ν2, ν1+ 2ν2+ 3ν3, and 4ν1+ ν3Bands

The very weak bands ν1+ 2ν2+ 3ν3and 4ν1+ ν3of16O3have been observed for the first time, using the Fourier transform spectrometer (FTS) of Reims and the usual experimental setup providing a large productp × lof approximately 38 Torr × 36 m. The upper levels of these A-type bands which are rather close in energy (they appear respectively at 5291.722 and 5307.790 cm−1) belong to two different sets of interacting polyads. To correctly reproduce the rotation–vibration energy levels and account for the observed perturbations, both bands are treated in a dyad approximation: the (123) state in the Coriolis resonance with the (330) state, and the (401) state in the Coriolis resonance with the (024) state. The assignments of the rotation–vibration levels of the (123) state are confirmed by measurements of line positions of the hot band ν1+ 2ν2+ 3ν3− ν2which has also been observed for the first time. The fits are very satisfactory: the r.m.s. deviation for 249 energy levels of the (123) state is 2.4 × 10−3cm−1and is 2.0 × 10−3cm−1for 266 levels of the (401) state. These r.m.s. are near experimental accuracy. Transition moments for the three observed bands are determined from measured line intensities.

Observation of population inversion in He I due to a double electron capture process

The first observation of inverted population density due to a double electron capture process has been performed for He plasma in contact with H 2 molecular gas in the TPD-II device. Recently, the process He 2+ + H 2 → He ∗ (1s3l) + 2H + has been found to have a very large cross-section (10 −15 cm 2 or more) in collisions at low energies (below 1 eV) due to an orbiting effect and accidental resonance. To see the population density of He I levels, a 2 m vacuum UV monochromator was used and resonance spectra ( np 1P → 1s 1S, n = 2,3) were measured. In contrast to the quick disappearance of the transition 2p 1P → 1s 1S of He I, the transition 3p 1P → 1s 1S has exhibited a noticeable increase of one order of magnitude associated with the injection of H 2 gas. As a result, population inversion between 2p 1P and 3p 1P was concluded to have occurred.

The effects of triplet perturbers on photophysical processes in the Ã1Au state of acetylene

Rotationally resolved excitation spectra of the acetylene à 1Au←X̃ 1∑g+ transition were recorded by measuring separately the total emission in two spectral regions: UV (200–400 nm) and NIR (0.9–1.5 μm). UV emission is due to Ã→X̃ transitions. The source of the NIR signal is suggested by the observation that the NIR:UV intensity ratio is approximately constant throughout the V02K01 subband. (V denotes the trans-bending mode.) This is consistent with C2H à 2Π→X̃ 2∑+ NIR transitions occurring as a result of resonance-enhanced two-photon excitation of predissociated Rydberg states. The V03K01 subband shows significant fluctuations in the NIR:UV intensity ratio. The increase in trans-bending excitation from 2ν3′ to 3ν3′ enables accidental resonances with background states (both triplet and S1) to alter the NIR emission intensity. Triplet perturbers can facilitate Tn (n&amp;gt;1)→T1 NIR transitions or modify the resonance enhancement of C2H Ã̃-state production. Extra lines in the V03K01 subband, due to an S1 anharmonic perturber, show dramatically diminished NIR but enhanced UV emission intensity. This indicates that the perturbing state is anomalously resistant to photodissociation.

Slow Fluorescence from Perturbed Glyoxal (S1) Levels Observed under Collision-Free Conditions

Jet-cooled trans-glyoxal molecules were excited in the 1S1 ← 1S0 transition using a pulsed, narrow-band (0.10-cm-1) dye laser. Collision-free conditions were ensured using a large nozzle−laser beam distance (27 mm) and low stagnation pressures (∼80 Torr). Spectrally unresolved fluorescence was observed with a photomultiplier viewing either the excitation region itself or a region of the molecular beam well downstream (46 mm) from the laser beam. Respectively, detection time windows of 0−6 and 30−80 μs after the laser pulse were used. In the latter case, the signal intensity was ∼2 × 10-3 of the former. This is at least 100 times greater than calculated from the known S1 glyoxal fluorescence lifetime (2.4 μs). However, only certain isolated rovibronic levels exhibit this anomalously slow fluorescence. The rotationally resolved excitation spectra of 17 S1 ← S0 bands were recorded both in the “slow” and “prompt” fluorescence mode. While the latter showed the usual dense line pattern, the former consisted of much fewer distinct lines, their number increasing with the excess vibrational energy. The corresponding emitting levels owe their long lifetime to the partial triplet character of their wave function, due to accidental resonance with T1-state levels. For the and S1 ← S0 bands, the rotational quantum numbers of the perturbed S1 levels were determined from a computer simulation. This is the first state-specific, collision-free observation of the previously predicted fluorescence lifetime lengthening through intramolecular S1/T1 coupling in small organic molecules. It could, in fact, be shown experimentally that in previous reports on the glyoxal phosphorescence excited in the S1 region, collisional effects must have obscured the behavior of the isolated molecules.

Ultraviolet photodissociation dynamics of formyl fluoride Part 1Competition between the decay channels for the Ãstate

T. W. R. Hancock and R. N. Dixon, J. Chem. Soc., Faraday Trans., 1997, 93, 2707 DOI: 10.1039/A701779K

Resonance enhanced multiphoton ionisation of the SiF radical: a reinvestigation

We report multiphoton ionisation spectra of ground (X 2Π) state SiF radicals, resonance enhanced at the two-photon energy by the v′ = 1 level of the C″ 2 Σ + excited state, at much higher resolution than hitherto. An earlier suggestion by Dulcey and Hudgens that the anomalous intensity of the C″-X (1,0) two-photon transition might be attributable to additional, accidental, resonance enhancement at the one-photon energy by levels of the A 2 Σ +, ν′ = 0 state is confirmed, and the double resonance enhancement analysed in detail. A similar accidental double resonance enhancement, involving levels of the A 2 Σ +, ν′ = 1 state at the one-photon energy, is proposed to explain the observed fragmentary pattern of lines associated with the C′ 2Π-X (5,0) two-photon transition.