Mulliken Bands 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.

Higher vibrational levels of the [formula omitted] state of dicarbon: New Mulliken bands

The Δv=+2 sequence of the Mulliken system of C2 is reported for the first time. Seven new levels of the D1Σu+ state of C2, v=5 through v=11, were found. The analysis reveals the rotational constants and origin of each band. The numbers are combined with the data reported for previously reported levels, v=0 through v=4, to generate updated Dunham parameters for the D1Σu+ state: ωe=1829.905(57) cm−1, ωexe=14.089(8) cm−1, ωeye=0 cm−1, ωeze=0.00124(3) cm−1, Be=1.83254(46) cm−1, αe=0.01909(18) cm−1and βe=-7.28(14)×10-5 cm−1. The determined equilibrium bond length Re=1.23804(28) Å. The fluorescence lifetimes of the new levels are also reported.

Electronic Spectroscopy of C2 in Solid Rare Gas Matrixes

Electronic spectroscopy of the C(2) molecule is investigated in Ar, Kr, and Xe matrixes in the 150-500 nm range. In the Ar matrix, the D ((1)Sigma(u)(+)) <-- ((1)Sigma(g)(+)) Mulliken band near 240 nm is the sole absorption in the UV range, whereas in the Kr matrix additional bands in the 188-209 nm range are assigned to the Kr(n)()(+)C(2)(-) <-- Kr(n)()C(2) charge-transfer absorptions. Because of the formation of a bound C(2)Xe species, the spectral observations in the Xe matrix differ dramatically from the lighter rare gases: the Mulliken band is absent and new bands appear near 300 and 423 nm. The latter is assigned to the forbidden B'((1)Sigma(g)(+)) <-- X ((1)Sigma(g)(+)) transition, but the origin of the former remains unclear. The spectral assignments are aided by electronic structure calculations at the MCSCF, CCSD(T), and BCCD(T) levels of theory and correlation consistent basis sets. A significant presence of multireference character of the C(2)Xe system was noted and a linear ground-state structure is predicted. The computational results contradict previous density functional studies on the same system.

Using photochemistry to explain the formation and observation of C 2 in comets

Laboratory data taken from the laser-induced fluorescence (LIF) spectrum of C 2(X 1 Σ g +) radicals produced during the 193 nm laser photolysis of C 2H 2 has been used to explain the band profiles of the Hubble Space Telescope (HST) on nucleus spectra of the C 2 Mulliken system (X 1 Σ g + ← D 1 Σ u +) in comet Hyakutake (C/1996 B2). The authors propose, using laboratory data and ab initio calculations, that photolysis of C 2H 2 in the laboratory and in comets occurs via a sequential mechanism, first producing C 2H and then C 2. The ab initio calculations were used to identify the two excited electronic states (2 2 Σ + and 2 2Π) of C 2H through which photodissociation in the second step proceeds. In both the laboratory and cometary studies, the photodissociation of C 2H appears to proceed via the same electronic surfaces. The C 2 Mulliken bands obtained in the laboratory and the HST observations of these bands are used to explain the internal energy distribution and origin of C 2(X 1 Σ g +) in comet Hyakutake. The off nucleus HST spectrum at 16 arcsec shows that the C 2 radicals are much colder vibrationally than they are on the nucleus. This vibrational cooling is inconsistent with literature reports and is discussed in the text.

C 2 formation in vibrationally excited CO

The formation of C 2 has been observed following the vibrational excitation of CO. Emission from four different electronic states has been observed: Swan bands (d 3 Π g → a 3 Π u); Deslandres-d'Azambuja bands (C 1 Π g → A 1 Π u); Fox-Herzberg bands (e 3 Π g → a 3 Π u) and a Mulliken band ( D 1Σ u + → X 1Σ g + ). The vibrational populations in the electronic states have been determined from low-resolution emission spectra. The time-resolved behaviour of the emissions indicates that the upper states close to the dissociation limit are populated first, while the lower electronic states are populated following a time delay. Formation mechanisms are discussed.

Laser-induced fluorescence study of laser sputtering of graphite

Pulsed laser irradiation of graphite surfaces has been known for some time to lead to the ejection of C, C 2, and C 3 neutrals as well as related ions. Since most relevant thermodynamic quantities are known, graphite represents an ideal system for further study. We report on the sputtering of pyrolytic, polycrystalline, and vitreous graphite by 20 ns pulses of laser light at 351 nm. The threshold energy density for sputtering is found to be 0.5 to 0.6 J/cm 2. At this fluence, the material removal rate is of the order of a monolayer/pulse. This is consistent with pulsed evaporation provided that the surface reaches a peak temperature of ~ 4000 K. The emitted particles are probed using laser-induced fluorescence (LIF). Kinetic (i.e. translational) energies are obtained by time-of-flight and correspond, for the lowest fluences, to ~ 4600 K. Rotational and vibrational distributions are obtained by analysis of the LIF spectra for the D 1Σ u +X 1Σ g + Mulliken bands of the dimer, C 2. Detailed analysis indicates a rotational temperature of 4100 ± 300 K and a vibrational temperature of 3650 ± 350 K. Since the temperatures are all similar at the lowest it is concluded that the laser sputtering of graphite involves thermally activated vaporization, i.e. is what is normally termed “thermal sputtering”. At higher fluences, the time-of-flight information appears to be significantly perturbed by Knudsen layer formation.

Spectrographic determination of traces of B, Be, Cd, Si and Co in U3O8 by the d.c. arc cathode-region method

The cathode region of the d.c. arc emission of U3O8 has been photographed (0.6 mm below the cathode) from 2,200 to 3,400 A, using Scribner-Mullin carrier distillation electrodes. Along with the elemental spectra of trace impurities in U3O8, a well resolved Mulliken band (1Σ u + → 1Σ g + ) of the C2 molecule has been observed in the 2,300 A region and its intensity has been found to follow the excitation conditions of the arc. Using some of the well resolved rotational lines in the Mulliken band of C2 molecule as internal standard lines, the trace impurities with analytical lines in 2,200 –2,500 A region (B, Be, Cd, Si and Co) have been satisfactorily determined. This method offers several advantages compared to the well established “carrier distillation” and also the “cathode region” method described by Avni as it has a “built-in” internal standard.

Carbon in Comet Bradfield 1979l

view Abstract Citations (39) References (10) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Carbon in Comet Bradfield 1979l A'Hearn, M. F. ; Feldman, P. D. Abstract Ultraviolet spectra of comet Bradfield obtained with IUE show a surprising absence of CO(+) despite the presence of CO2(+), and they also show, for the first time, the unambiguous presence of the Mulliken bands of C2. With the use of simultaneous ground-based data, a value of 0.00001 is derived for the transition moment of the a 3 Pi-X 1 Sigma g + transition of C2. The anomalous CO(+)/CO2(+) ratio can be accounted for by the different scale lengths of the species rather than by an abundance difference. Using the model of Feldman (1978), the upper limit for CO(+) is found to be consistent with the observed abundance of CO. On that same model, however, CO cannot produce nearly as much atomic carbon as is observed. Publication: The Astrophysical Journal Pub Date: December 1980 DOI: 10.1086/183429 Bibcode: 1980ApJ...242L.187A Keywords: Abundance; Carbon; Comets; Ultraviolet Spectra; Brightness; Carbon Dioxide; Carbon Monoxide; Iue; Positive Ions; Astronomy full text sources ADS | data products MAST (1)

Production of carbon, sulfur, and CS in Comet West

view Abstract Citations (77) References (31) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Production of carbon, sulfur, and CS in Comet West Smith, A. M. ; Stecher, T. P. ; Casswell, L. Abstract Results from ultraviolet spectrographic observations of Comet West (1975n) in the 1620-3960 A wavelength range are presented. Objective grating imagery of the comet reveals for the first time emission from sulfur atoms and CS molecules as well as previously observed lines or bands in carbon atoms and CO(+), CO2(+) and OH molecules. Some evidence exists for emission in the C2 Mulliken bands and in the CO Cameron bands. On the basis of the observations, production rates of C (ground state), C(1D), S, and CS are inferred to be 3.8 x 10 to the 29th per sec, 4.4 x 10 to the 29th per sec, 1.7 x 10 to the 28th per sec, and greater than approximately 3.1 x 10 to the 27th per sec, respectively. Publication: The Astrophysical Journal Pub Date: November 1980 DOI: 10.1086/158473 Bibcode: 1980ApJ...242..402S Keywords: Astronomical Spectroscopy; Carbon Compounds; Sulfides; Sulfur; West Comet; Molecular Ions; Rocket Sounding; Spectral Bands; Ultraviolet Astronomy; Astrophysics full text sources ADS |

Some comments on the calculations of oscillator strengths from approximate wave functions

Energy factors, operators, and wave functions used in the calculation of electronic oscillator strengths are critically examined. The role of electron correlation in such calculations is emphasized. Oscillator strengths for the Mulliken bands of C 2 ( 1 Σ g +- 1 Σ u + ) have been calculated from wave functions including configuration interaction.