Perspective: C60+ and laboratory spectroscopy related to diffuse interstellar bands.

In the last 30 years, our research has focused on laboratory measurements of the electronic spectra of organic radicals and ions. Many of the species investigated were selected based on their potential astrophysical relevance, particularly in connection with the identification of appealing candidate molecules for the diffuse interstellar absorptions. Notably, carbon chains and derivatives containing hydrogen and nitrogen atoms in their neutral and ionic forms were studied. These data could be obtained after developing appropriate techniques to record spectra at low temperatures relevant to the interstellar medium. The measurement of gas phase laboratory spectra has enabled direct comparisons with astronomical data to be made and though many species were found to have electronic transitions in the visible where the majority of diffuse bands are observed, none of the absorptions matched the prominent interstellar features. In 2015, however, the first carrier molecule was identified: C60+. This was achieved after the measurement of the electronic spectrum of C60+-He at 6K in a radiofrequency ion trap.

Electronic Spectra of Protonated Fluoranthene in a Neon Matrix and Gas Phase at 10 K.

Four electronic systems with origin bands at 759.5, 559.3, 476.3, and 385.5 nm are detected in a 6 K neon matrix following deposition of mass-selected protonated fluoranthene C16H11(+) produced from a reaction of neutral vapor and ethanol in a hot-cathode ion source. Two cationic isomers are identified as the carriers of these band systems. The 559.3, 476.3, and 385.5 nm absorptions are assigned to 4,3,2 (1)A' ← X (1)A' transitions of isomer E(+) (γ-) and the 2 (1)A' ← X (1)A' system at 759.5 nm is of isomer C(+) (α-) of protonated fluoranthene on the basis of theoretical predictions. The electronic spectrum of E(+) was also recorded in the gas phase using a resonant 1 + 1 two-photon excitation-dissociation technique in an ion trap at vibrational and rotational temperatures of 10 K. The 3,2 (1)A' ← X (1)A' transitions have origin band maxima at 558.28 ± 0.01 and 474.92 ± 0.01 nm. Both the 2 (1)A' and 3 (1)A' excited states have a distinct vibrational pattern with lifetimes on the order of 1 ps.

Laboratory confirmation of C60(+) as the carrier of two diffuse interstellar bands.

The diffuse interstellar bands are absorption lines seen towards reddened stars. None of the molecules responsible for these bands have been conclusively identified. Two bands at 9,632 ångströms and 9,577 ångströms were reported in 1994, and were suggested to arise from C60(+) molecules (ref. 3), on the basis of the proximity of these wavelengths to the absorption bands of C60(+) measured in a neon matrix. Confirmation of this assignment requires the gas-phase spectrum of C60(+). Here we report laboratory spectroscopy of C60(+) in the gas phase, cooled to 5.8 kelvin. The absorption spectrum has maxima at 9,632.7 ± 0.1 ångströms and 9,577.5 ± 0.1 ångströms, and the full widths at half-maximum of these bands are 2.2 ± 0.2 ångströms and 2.5 ± 0.2 ångströms, respectively. We conclude that we have positively identified the diffuse interstellar bands at 9,632 ångströms and 9,577 ångströms as arising from C60(+) in the interstellar medium.

Gas-phase electronic transitions of C17H12N⁺ at 15 K.

The electronic spectrum of C17H12N(+), phenanthrene with a side chain, was measured in the gas phase at a vibrational and rotational temperature of ∼15 K in an ion trap using a resonant multiphoton dissociation technique. The C17H12N(+) structure was produced in a chemical ionization source and identified by a comparison with theoretical calculations of stable structures and excitation energies. The (3), (2), (1) (1)A ← X (1)A electronic transitions of this nitrogen-containing aromatic species with 30 atoms have origin band maxima at 23,586 ± 1 cm(-1), 16,120 ± 50 cm(-1), and 14,519 ± 30 cm(-1). Distinct vibrational structure in the (3) (1)A state is observed, and assignments are made. Astronomical aspects are considered.

(1) (1)A' ← X (1)A' Electronic Transition of Protonated Coronene at 15 K.

The electronic spectrum of protonated coronene in the gas phase was measured at vibrational and rotational temperatures of ∼15 K in a 22-pole ion trap. The (1) (1)A' ← X (1)A' electronic transition of this larger polycyclic aromatic hydrocarbon cation has an origin band maximum at 14 383.8 ± 0.2 cm(-1) and shows distinct vibrational structure in the (1) (1)A' state. Neither the origin nor the strongest absorptions to the blue coincide with known diffuse interstellar bands, implying that protonated coronene is not a carrier.

Electronic spectroscopy of carbon chains and rings of astrophysical interest.

This perspective is concerned with laboratory measurements of the electronic spectra of carbon chains, rings, and their ions, including derivatives terminated by hydrogen and nitrogen atoms. The selected-species have relevance to astronomical observations through diffuse clouds, absorption features known as diffuse interstellar bands (DIBs). Two indications to decide which molecules should be studied are the observations of polar carbon chains in dense clouds by rotational spectroscopy and the knowledge that a certain number of these have electronic transitions in the DIB region. This information has been obtained initially by measurements of the electronic absorptions in 6 K neon matrixes using mass-selection. This was followed by the gas-phase observations using cavity ringdown and resonance enhanced techniques in combination with pulsed-supersonic discharge sources or via laser vaporization. The gas-phase spectra were then compared with DIB data, all with negative results, except for the detection of C3, but leading to upper limits of their column densities <10(12) cm­2. By reference to mm-wave absorption measurements in the diffuse medium, it is shown that, although species such as H2C3 are present there, the product of the expected column densities and oscillator strength of the transitions will lead to only very weak DIBs. The significant conclusion is that carbon chains and their derivatives containing hydrogen or nitrogen comprising up to a dozen atoms cannot be responsible for stronger DIBs. However, chains with an odd-number of carbon atoms, C17, C19, ···, have very intense transitions in the region above 4400 Å and remain attractive candidates. An uncertainty is the excited electronic state lifetime; if this is less than 70 fs, then the resulting absorptions would be too broad to be astronomically relevant. The electronic absorptions of some of the species studied bear a striking resemblance to DIB data. The two peaked rotational contour of the origin band in the electronic transition of dicyanoacetylene cation is superimposable on a DIB absorption when shifted by 1 Å. The band profiles of cyclic C18 at 100 or 20 K are similar to DIBs but differ in wavelength. This suggests that another set of potential candidates are the carbon rings of sizes up to a hundred of atoms, including ions and heavy atoms, with the requirement of a large oscillator strength. Observations on the absorptions of propadienylidene C3H2 and C60+ are discussed.

Electronic absorption spectrum of triacetylene cation for astronomical considerations.

The A(2)Πg ← X(2)Πu electronic transition (4800-6000 Å) of triacetylene cation was measured in an ion trap, where the vibrational and rotational degrees of freedom were equilibrated to 25 K. The rotational profile of the origin band is predicted by a collisional-radiative rate model under conditions expected in diffuse interstellar clouds. Variation in the density of the surrounding gas, rotational temperature, and velocity dispersion are taken into account.

The electronic spectrum of Si3 I: triplet D(3h) system.

We report the measurement of a jet-cooled electronic spectrum of the silicon trimer. Si(3) was produced in a pulsed discharge of silane in argon, and the excitation spectrum examined in the 18 000-20 800 cm(-1) region. A combination of resonant two-color two-photon ionization (R2C2PI) time-of-flight mass spectroscopy, laser-induced fluorescence/dispersed fluorescence, and equation-of-motion coupled-cluster calculations have been used to establish that the observed spectrum is dominated by the 1(3)A(1)" - ã (3)A(2)' transition of the D(3h) isomer. The spectrum has an origin transition at 18,600 ± 4 cm(-1) and a short progression in the symmetric stretch with a frequency of ∼445 cm(-1), in good agreement with a predicted vertical transition energy of 2.34 eV for excitation to the 1(3)A(1)" state, which has a calculated symmetric stretching frequency of 480 cm(-1). In addition, a ∼505 cm(-1) ground state vibrational frequency determined from sequence bands and dispersed fluorescence is in agreement with an earlier zero-electron kinetic energy study of the lowest D(3h) state and with theory. A weaker, overlapping band system with a ∼360 cm(-1) progression, observed in the same mass channel (m/z = 84) by R2C2PI but under different discharge conditions, is thought to be due to transitions from the (more complicated) singlet C(2v) ground state ((1)A(1)) state of Si(3). Evidence of emission to this latter state in the triplet dispersed fluorescence spectra suggests extensive mixing in the excited triplet and singlet manifolds. Prospects for further spectroscopic characterization of the singlet system and direct measurement of the energy separation between the lowest singlet and triplet states are discussed.

Two-color photodetachment study of the A3Π-X3Σ- origin band of C5H-.

The origin band in the electronic transition to the dipole bound excited state of C(5)H(D)(-) anions was measured using two-color photodetachment spectroscopy. The rotational analysis of the partially resolved contour is consistent with a linear structure of the anion in both the ground X(3)Σ(-) and excited A(3)Π electronic states, in contrast with an earlier interpretation. The following spectroscopic constants are inferred for C(5)H(-): T(00) = 19248.0(1), B' = 0.0835(1), B'' = 0.0826(2), A'(SO) = -11.96(1), λ(SS)' = 1.97(8) λ(SS)'' = 0.24(15). Ab initio calculations at the UHF-MP2 level support the conclusion that C(5)H(-) is linear in the ground state. The experimentally determined ground state rotational constant can be used in the search for the millimeter wave spectrum of C(5)H(-).

D2Pi(u), C2Pi(u) <-- X2Pi(g) electronic transitions of NCCN+.

The electronic absorption spectrum of NCCN(+) in the gas phase was measured at approximately 15 K in a 22-pole ion trap. The spectra show two band systems assigned to the C(2)Pi(u)-X(2)Pi(g) and D(2)Pi(u)-X(2)Pi(g) transitions with origin band maxima at 17,363 (3) and 33,409 (5) cm(-1), respectively. Both absorptions show distinct vibrational structure with progressions in nu(2) as well as combinations of double quanta excitations in nu(4) and nu(5). Rotational structure of the 0(0)(0) bands could not be resolved, which indicates that the C(2)Pi(u) and D(2)Pi(u) states have a lifetime on the order of a hundred femtoseconds because of fast intramolecular processes.

Gas phase electronic spectrum of T-shaped AlC2 radical.

Gas phase electronic transitions for the C 2B2<--X 2A1 and D 2B1<--X 2A1 band systems of T-shaped AlC2 (C2v) radical have been measured in the 345-475 nm range. Vibrational analyses of both band systems are reported. Simulation of several rotationally resolved bands confirms previously obtained rotational parameters for the C 2B2 state. The radical is produced by ablating an aluminum rod in the presence of acetylene gas. The resulting supersonic molecular beam is probed using both mass-selective resonant two-color two-photon ionization and laser induced fluorescence. Ab initio calculations and vertical electronic excitation energies help the assignment. Vibrational frequencies for the X 2A1, C 2B2, and D 2B1 states have been determined. Rotational analysis of a number of bands yields spectroscopic constants for one vibronic state in the C 2B2 manifold and the origin band of the D 2B1<--X 2A1 system.

B (2)Sigma(u)+ <-- X (2)Pi(g) electronic spectrum of NCCN+ in the gas phase.

The B (2)Sigma(u)(+) <-- X (2)Pi(g) absorption spectrum of NCCN(+) in the gas-phase was observed using a two-color, two-photon photodissociation technique. This was measured at approximately 20 K in a 22-pole ion trap with laser bandwidths of less than a cm(-1). The spectrum shows distinct vibrational structure, with the origin band near 11,253 cm(-1), and the excitation of four normal modes in the excited state. The rotational structure of the 0(0)(0) band in the gas phase could not be resolved, indicating that the B (2)Sigma(u)(+) state has a lifetime of a few picoseconds because of a fast intramolecular process.

Gas-phase electronic spectrum of the C14 ring.

The electronic spectrum of a cyclic C(14) in the visible range has been detected in the gas phase by a mass selective resonant two-color two-photon ionization technique coupled to a laser ablation source. Absorption is localized in the 19 000 to 20 000 cm(-1) region and appears as a dozen of 3-7 cm(-1) narrow peaks belonging to one or two close-lying electronic states. Bands have structures which for the narrowest ones is likely to be the rotational profile contour. The spectrum is attributed to a cyclic form of C(14) based on time-dependent density-functional calculations and reactivity with H(2). The spectral pattern differs from that previously seen in the larger C(4n+2) member rings, C(18) and C(22), indicating some sort of a structural crossover.

Gas phase electronic spectra of the carbon chains C5, C6, C8, and C9.

Three electronic absorption systems for C5 at 511, 445, and 232 nm and one for C6, C8, and C9 centered at 228, 259, and 288 nm have been observed in the gas phase. The C5 chain was produced in both discharge and ablation sources and detected using resonant two-color two-photon ionization spectroscopy involving 10.5 eV photons. The decay of the excited singlet electronic states indicates fast intramolecular processes on a subpicosecond time scale. The internal energy is assumed to be trapped in a triplet state for at least 15 micros. Hole-burning experiments on the 2 (3)Sigma(u)- <-- X (3)Sigma(g)- transition of C6, C8, and (1)Sigma(u)+ <-- X (1)Sigma(g)+ of C9 confirm the predissociative nature of the excited electronic states.

Gas-phase electronic spectra of C18 and C22 rings.

The electronic spectra of C(18) and C(22) in the 15 150-36 900 cm(-1) range have been detected in the gas phase by a mass-selective resonant two-color two-photon ionization technique coupled to a laser ablation source. The spectra were assigned to several electronic systems of monocyclic cumulenic isomers with a D(9 h) symmetry for C(18) and D(11 h) for C(22), based on time-dependent-density-functional calculations and reactivity with respect to H(2). The best cooling conditions were achieved with Kr as the buffer gas, and the origin of the A(1)A(2) (")<--X(1)A(1) (') transition of C(18) at 592.89 nm shows a pair of 1 cm(-1) broadbands spaced by 1.5 cm(-1). The next electronic transitions exhibited much broader, approximately 30 (in the visible) to 200 cm(-1) (in ultraviolet range), features. The spectrum of C(22) exhibits an absorption pattern similar to C(18), except that the narrow features to the red are missing; the oscillator strength of the A<--X transition is predicted to be low.

Gas phase electronic spectra of two C5H5 radical isomers.

Gas phase electronic spectra of two C5H5 radical isomers have been observed in the 440-470 nm spectral region. The technique was a mass selective resonant two-color two-photon ionization coupled to a supersonic plasma source. Structures, relative energies and vertical electronic transition energies of six isomers of C5H5 have been calculated. Based on an analysis of the rotational profiles of the observed bands and theoretical calculations, the spectra are assigned as the A 2A" <-- X 2A" electronic transition of isomer 1 and A 2A2 X 2B1 of 6 with origin band at 461.8 nm and 456.1 nm, respectively. Isomer 1, 1,3-vinylpropargyl, has Cs symmetry, while 6, a planar zig-zig chain with one hydrogen on each carbon, has C2v symmetry.

Gas phase detection of cyclic B3: 2(2)E' <-- X2A1' electronic origin band.

The rotationally resolved origin band in the 2(2)E'<--X2A1' electronic spectrum of cyclic B3 has been observed by cavity ring down spectroscopy in the gas phase. The B3 molecule was generated in a supersonic planar plasma containing decaborane (B10H14) and neon as a carrier gas. The rotational structure pattern is that of a cyclic molecule. It is analyzed assuming an equilateral triangle in both electronic states. The band origin is determined to be 21 853.52 cm(-1), and the bond lengths 1.603 77(106) A in the ground and 1.619 07(96) A in the excited electronic state are inferred from analysis of the rotational structure.

Rotationally resolved infrared spectrum of the charge transfer complex [Ar-N2]+.

Difficulties in preparing cluster ions for spectroscopic studies have limited our understanding of intermolecular forces in charged complexes that are typical of many reactive intermediates. Here, the infrared spectrum of the charge transfer complex [Ar-N2]+, recorded in a supersonic planar plasma with a tunable diode laser spectrometer, is presented. More than 70 adjacent rovibrational transitions were measured near 2272 wave numbers and assigned to the molecular nitrogen stretching fundamental in the 2Sigma+ ground state of [Ar-N2]+. The accurate structural parameters that were determined confirm a linear structure and show that the major part of the charge is located at the argon atom. The latter result is surprising and implies a charge switch of the cationic center upon complexation.

Pediatric melanoma: confirming the diagnosis with sentinel node biopsy.

Many pediatric melanoma lesions present at a more advanced stage than those in the adult population. Clinical and histological melanoma mimics, including a subset of Spitz nevi, are difficult to discriminate from melanoma. When dealing with a childhood melanoma, the clinician is likely to be faced with a thick lesion, and one in which the actual diagnosis may even be in doubt. There is a paucity of data to guide the physician in his management of melanoma in this age group, particularly with respect to node status and adjuvant therapy. The authors present two cases of pediatric melanoma in which the novel use of sentinel node biopsy helped confirm the diagnosis of melanoma, determined the need for full lymph node dissection, and guided the use of adjuvant interferon therapy.

Optoacoustic trace-gas monitoring with near-infrared diode lasers.

An inexpensive resonant optoacoustic monitoring system using near-infrared laser diodes was developed. It was demonstrated that wavelength modulation at the resonance frequency of the cell provides a superior signal-to-noise ratio compared with amplitude modulation and eliminates background drifts and fluctuations. The system was tested out on ammonia. Its sensitivity is 8 parts in 10(9) (S/N = 1) at atmospheric pressure, which corresponds to a minimum detectable absorption coefficient of approximately 3.5 × 10(-11) cm(-1) W(-1). The pressure dependence of the optoacoustic resonance was also investigated. The monitor can be used as a continuous flow-through system up to a flow rate of approximately 3.5 L/min.