Electronic Origin Band Research Articles

The EDIBLES survey

Context. Small linear carbon chain radicals such as C2 and C3 act as both the building blocks and dissociation fragments of larger carbonaceous species. Their rotational excitation traces the temperature and density of local environments. However, these homo-nuclear di- and triatomic species are only accessible through their electronic and vibrational features because they lack a permanent dipole moment, and high signal-to-noise ratio data are necessary as the result of their generally low abundances in the interstellar medium (ISM). Aims. In order to improve our understanding of small carbonaceous species in the ISM, we carried out a sensitive survey of C2 and C3 using the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) dataset. We also expanded our searches to C4, C5, and the 13C12C isotopologue in the most molecule-rich sightlines. Methods. We fitted synthetic spectra generated following a physical excitation model to the C2 (2-0) Phillips band to obtain the C2 column density (N) as well as the kinetic temperature (Tkin) and number density (n) of the host cloud. The C3 molecule was measured through its à − $ \tilde X$ (000-000) electronic origin band system. We simulated the excitation of this band with a double-temperature Boltzmann distribution. Results. We present the largest combined survey of C2 and C3 to date in which the individual transitions can be resolved. In total, we detected C2 in 51 velocity components along 40 sightlines, and C3 in 31 velocity components along 27 sightlines. Further analysis confirms the two molecules are detected in the same velocity components. We find a very good correlation between N(C2) and N(C3) with a Pearson correlation coefficient r = 0.93 and an average N(C2)/N(C3) ratio of 15.5± 1.4. A comparison with the behaviour of the C2 diffuse interstellar bands (DIBs) shows that there are no clear differences among sightlines with and without detections of C2 and C3. This is in direct contrast to the better-studied non-C2 DIBs, which have reduced strengths in molecule-rich environments, consistent with the idea that the C2 DIBs are indeed a distinguishable DIB family. We also identify, for the first time, the Q(2), Q(3), and Q(4) transitions of the 13C12C (2-0) Phillips band in the stacked average spectrum of molecule-rich sightlines, and estimate the isotopic ratio of carbon 12C/13C to be 79±8, consistent with literature results. At this stage it is not yet possible to identify these transitions in individual sightlines. Our search for the C4 and C5 optical bands was unsuccessful; even in stacked spectra no unambiguous identification could be made.

∧-type doublets and lifetime broadening in the B2Π–X2Π electronic origin band of C6H

We present a high-resolution study on the B2Π-X2Π electronic origin band spectrum of the C6H radical with its center at ~18990 cm-1 The C6H radicals are produced in a pulsed supersonic slit-jet discharge source and detected in direct absorption by sensitive cavity ring-down spectroscopy. By using a home-made narrow linewidth nanosecond pulsed laser source, the experimental spectrum allows to resolve the ∧-type doublet fine structures in high-J rotational lines. A set of improved B2Π state constants, including the ∧-type doubling parameters of p′ = −1.16(9)x10−3 cm−1q′ = -1.22(7)x10−4 cm−1 are determined from analysis of the experimental spectrum. An analysis on the spectral line profile has resulted in a significant linewidth broadening of ~0.05±0.01 cm−1, corresponding to a B2Π state lifetime of ~100±20 ps. The short lifetime of the excited B2Π state is proposed due to fast internal vibronic couplings to high-lying vibronic levels of lower electronic states.

Rotationally resolved electronic spectroscopy of 6-methylindole: Structures, transition moments, and permanent dipole moments of ground and excited singlet states

The rotationally resolved electronic spectrum of the S1←S0 electronic origin band of 6-methylindole (6-MI) has been measured in a molecular beam and has been fit to rigid asymmetric rotor Hamiltonians in both electronic states using evolutionary strategies. Rotational constants, dipole moments in both electronic states, and orientation of the transition dipole moment in the inertial frame of the molecule have been determined and compared to the results of ab initio calculations. Both the values of the permanent dipole moments as of the transition dipole moment point to an Lb-state as lowest electronically excited state in 6-MI.

Probing the Endo/Exo Isomeric Variants of a Binary Complex of Cyclopropylamine with p-Fluorophenol by LIF Spectroscopy: A Comparative Study with Ammonia Complex.

Conformational isomers of an O-H···N hydrogen-bonded binary complex between para-fluorophenol (pFP) and a nonrigid primary amine base, cyclopropylamine (CPA), have been probed by means of laser-induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Two closely spaced electronic origin bands have been identified in the measured LIF excitation spectrum, and their assignments have been corroborated by making comparisons with the spectra of the parent pFP-NH3 complex recorded under the same expansion condition. The observation is consistent with the presence of endo and exo isomeric variants of the complex predicted by electronic structure theory methods, and the endo isomer is stabilized by ∼2 kcal/mol additionally owing to the formation of a C-H···O and a C-H···π type of weak hydrogen bonds between the two moieties. In the fluorescence excitation (FE) spectrum, the low-frequency bands for different intermolecular modes gain substantial intensity, and this spectral feature is in contrast to that of the pFP-NH3 complex. The Franck-Condon intensity of the bands has been simulated invoking Duschinsky rotation scheme, taking into consideration the ground- and excited-state geometries.

The C ̃1Bu ← X ̃1Σg+ Transition of Ethanedinitrile (C2N2): Assignment of the Electronic State Origin and Vibrational Bands.

An analysis is presented for the vacuum ultraviolet C2N2 C̃1Bu ← X̃1Σg+ transition. Contrary to previous assumptions this is found to be a linear to bent transition with a trans-bent C1Bu excited electronic state. The band origin is at 59 840 cm-1. Vibrational bands between 170 and 145 nm are assigned. Vibrational frequencies for the C̃1Bu state are found to be ν1 = 2055 ± 20 cm-1 for the symmetric CN stretch, ν2 = 979 ± 20 cm-1 for the symmetric CC stretch, ν3 = 542 ± 10 cm-1 for the trans-bend, ν4 = 131 ± 10 cm-1 for the out-of-plane torsional bend, and ν6 = 141 ± 10 cm-1 for the cis-bend.

Rotationally resolved spectrum of the B1A′ – X1A′ 000 band of CuSH

We present a high resolution spectroscopic study on the B1A′ – X1A′ 000 electronic origin band of CuSH near 434 nm. The CuSH molecules are produced by a corona discharge of a H2S/Ar gas mixture with copper needle electrodes in a supersonic jet expansion. The high-resolution spectrum of the B1A′ – X1A′ 000 band of jet-cooled CuSH are recorded by laser induced fluorescence with a spectral resolution of ∼0.02 cm−1. From a detailed rotational analysis of the experimental spectrum, accurate spectroscopic constants of the B1A′ state are determined for both 63CuSH and 65CuSH isotopologues.

The B2Π–X2Π electronic origin band of 13C6H

The rotationally resolved spectrum of the B2Π–X2Π electronic origin band transition of 13C6H is presented. The spectrum is recorded using cavity ring-down spectroscopy in combination with supersonic plasma jets by discharging a 13C2H2/He/Ar gas mixture. A detailed analysis of more than a hundred fully-resolved transitions allows for an accurate determination of the spectroscopic parameters for both the ground and electronically excited state of 13C6H.

Identification of tyrosine-phosphorylated peptides using cold ion spectroscopy.

The accurate and unambiguous detection of post-translational modifications in proteins and peptides remains a challenging task. We report here the use of cold ion spectroscopy for the identification of phosphorylated tyrosine residues in peptides. This approach employs the wavelength-specific UV fragmentation of cryogenically cooled protonated peptides in the gas phase. In addition to the appearance of specific photofragments, the phosphorylation of tyrosine induces large spectral shifts of the peptide electronic band origins. Quantum chemical calculations and experiments together suggest a certain generality of the use of such shifts in the spectroscopic identification of phosphotyrosines. The enhanced selectivity offered by the joint application of wavelength-specific fragmentation and mass spectrometry of cold molecules can also be used in the identifications of aromatic residues in protonated peptides and, potentially, of other UV-absorbing groups in a variety of large polyatomic ions.

The electronic spectra of long carbon-chain (13)C2nH/D molecules for ()

Unresolved electronic origin band transitions have been measured for the even-numbered linear carbon-chain radicals (13)C2nH/D with . Spectra of the fully 13C substituted 13C8H/D and 13C10H/D isotopologues are presented for the first time. 12C10H/D and 12C12H/D spectra are presented with improved signal-to-noise ratio to realise a systematic study of the band profiles. Despite the fact that individual rotational transitions cannot be resolved, it is possible to derive effective values for the upper state lifetime. The obtained natural lifetime line broadening parameters of 0.8 ± 0.1 cm−1 for n = 4, 4.0 ± 0.5 cm−1 for n = 5 and 4.8 ± 0.5 cm−1 for n = 6 indicate that internal conversion processes occur in these carbon-chain molecules.

Rotationally resolved spectra of the 4051 Å comet band of C3 for all six 12C and 13C isotopologues

Rotationally resolved absorption spectra of the A∼1Πu–X∼1Σg+ origin band of the tricarbon molecule (C3) are reported for 12C3 and its 13C-substitued isotopologues. The 4051Å electronic transition, also identified in cometary tails and in translucent interstellar clouds, has been measured using cavity ring-down spectroscopy and a supersonically expanding planar plasma. Previous spectroscopic studies of 12C3 are extended and the electronic origin band is re-investigated including a perturbation analysis of the upper state. The rotational analysis of spectra originating from 13C mono- and di-substituted C3 yields accurate spectroscopic parameters for all six isotopologues, including ground state molecular constants for 13C12C13C and 12C13C13C.

Building Up Water-Wire Clusters: Isomer-Selective Ultraviolet and Infrared Spectra of Jet-Cooled 2-Aminopurine (H2O)n, n = 2 and 3

2-Aminopurine (2AP) is an adenine analogue with a high fluorescence quantum yield in water solution, which renders it a useful real-time probe of DNA structure. We report the ultraviolet (UV) and infrared (IR) spectra of size-selected and jet-cooled 9H-2AP·(H2O)n clusters with n = 2 and 3. Mass- and species-specific UV/UV holeburning spectroscopy allows to separate the UV spectra of four cluster isomers in the 31,200–33,000 cm(–1) spectral region with electronic band origins at 31339, 31450, 31891, and 32163 cm(–1). Using IR/UV depletion spectroscopy in combination with B3LYP calculated harmonic vibrational frequencies, the H-bonding topologies of two isomers of the n = 2 and of two isomers of the n = 3 cluster are identified. One n = 2 isomer (denoted 2A) forms a water dimer chain between the N9H and N3 atoms at the sugar-edge site, the other isomer (denoted 2D) binds one H2O at the sugar-edge site and the other at the trans-amino site between the N1 atom and the NH2 group. For 2-aminopurine·(H2O)3, one isomer (denoted 3A) forms an H-bonded water wire at the sugar-edge site, while isomer 3B accommodates two H2O molecules at the sugar-edge and one at the trans-amino site. The approximate second-order coupled cluster (CC2) method predicts the adiabatic S1 ← S0 transitions of 9H-2-aminopurine and six water cluster isomers with n = 1–3 in very good agreement with the experimental 0(0)(0) frequencies, with differences of <0.6%. The stabilization of the S1(ππ*) state of 2-aminopurine by water clusters is highly regiospecific: Isomers with one or two H2O molecules H-bonded in the trans-amino position induce large spectra red shifts, corresponding to 1ππ* state stabilization of 10–12 kJ/mol, while water-wire cluster solvation at the sugar-edge leads to much smaller stabilization. The evolution of the IR spectra of the water-wire clusters with n = 1–3 that are H-bonded to the sugar-edge site is discussed. Qualitatively different regions (denoted I to IV) can be attributed to the different free and H-bonded OH, NH, NH2 and OH···OH water-wire stretch vibrations.

Ground and Electronically Excited Singlet State Structures of the syn and anti Rotamers of 5-Hydroxyindole

The electronic origin bands A and B of 5-hydroxyindole were measured using rotationally resolved electronic spectroscopy. From comparison of the experimental rotational constants to the results of ab initio calculated structures, we could make the assignment of band A being due to the syn conformer and of band B being due to the anti conformer. These conformers, which differ in the orientation of the hydroxy group with respect to the rest of the molecule, have considerably different S1 state life times. The most probable explanation for this surprising finding is a different conical intersection of the ππ* states of both conformers with the repulsive πσ* state.

Spin-orbit Splitting and Lifetime Broadening in theA2Δ Electronic State ofl-C5H

Optical absorption bands at ∼18772 and ∼18807 cm−1, previously assigned to A2Δ-X2Π electronic origin band transitions of the linear carbon-chain radicals C5H and C5D, respectively, have been reinvestigated. The spectra have been recorded in direct absorption applying cavity ring-down spectroscopy to a supersonically expanding acetylene/helium plasma. The improved spectra allow deducing a l-C5H upper state spin-orbit coupling constant A′=−0.7(3) cm−1 and a A2Δ lifetime of 1.6±0.3 ps.

Fluorescence Spectroscopic and Time-Dependent Density-Functional Theory Studies of Diphenylsilane

We investigated fluorescence and fluorescence excitation of diphenylsilane (DPS) in a solution and molecular beams in combination with the aid of the DFT method. When the molecule was photoexcited at 250 nm in a cyclohexane solution, normal and excimer fluorescences were observed in the ranges of 260-320 and 330-450 nm, respectively. The fluorescence excitation spectrum indicates that the channel leading to the intramolecular excimer formation is not efficient in comparison with the normal fluorescence. Vibrationally resolved fluorescence excitation spectra were measured for the DPS molecules cooled in pulsed supersonic expansion of He in the range 262.2-271.7 nm, in which we can see several electronic excitation spectra exhibiting the electronic band origins. We found that the simulated absorption spectrum based on the time-dependent densityfunctional theory calculations accords well with the absorption spectrum.

Structure and intermolecular vibrations of 7-azaindole-water 2:1 complex in a supersonic jet expansion: Laser-induced fluorescence spectroscopy and quantum chemistry calculation#

Laser-induced fluorescence spectra of a 2:1 complex between 7-azaindole and water, known as ‘non-reactive dimer’ of the molecule, have been measured in a supersonic jet expansion. The dispersed fluorescence spectrum of the electronic origin band of the complex shows a very large number of low-frequency vibrational features corresponding to different intermolecular modes of the complex in the ground electronic state. Geometries of several possible isomeric structures of the complex and their vibrational frequencies at harmonic approximation were calculated by electronic structure theory method at MP2/6-31G** level. An excellent agreement is observed between the measured and calculated intermolecular vibrational mode frequencies for the energetically most favoured structure of the complex, where the water molecule is inserted within one of the two N⋯H–N hydrogen bonds of the 7AI dimer.

Structure of 7-Azaindole···2-Fluoropyridine Dimer in a Supersonic Jet: Competition between N–H···N and N–H···F Interactions

In the present work, we have investigated the structure of 7-azaindole···2-fluoropyridine dimer in a supersonic jet by employing resonant two photon ionization (R2PI), IR-UV, and UV-UV double resonance spectroscopic techniques combined with quantum chemistry calculations. The R2PI spectrum of the dimer is recorded by electronic excitation of the 7-azaindole moiety, and a few low frequency intermolecular vibrations of the dimer are clearly observed in the spectrum. The electronic origin band of the dimer is red-shifted by 1278 cm(-1) from the S(1) ← S(0) origin band of 7-azaindole monomer. The presence of a single conformer of the dimer is confirmed by IR-UV and UV-UV hole-burning spectroscopic techniques. RIDIR (Resonant ion dip infrared) spectrum of the dimer shows a red-shift of 265 cm(-1) in the N-H stretching frequency with respect to that of the 7-azaindole monomer. Two planar double hydrogen bonded cyclic structures of the dimer have been predicted from DFT calculations. Comparison of experimental and theoretical N-H stretching frequencies confirms that the observed dimer is stabilized by N-H···N and C-H···N hydrogen bonding interactions. The less stable conformer with N-H···F and C-H···N interactions are not observed in the experiment. The competition between N-H···N and N-H···F interactions in the two dimeric structures are discussed from natural bond orbital (NBO) analysis. The current results demonstrate that fluorine makes a hydrogen bond of intermediate strength through cooperative interaction of another hydrogen bond (C-H···N) present in the dimer, although fluorine is believed to be very weak hydrogen bond acceptor.

C6H and C6D: Electronic spectra and Renner-Teller analysis

Rotationally resolved spectra of the B(2)Π - X(2)Π 0(0)(0) electronic origin bands and 11(1)(1) μ(2)Σ-μ(2)Σ vibronic hot band transitions of both C(6)H and C(6)D have been recorded in direct absorption by cavity ring-down spectroscopy through a supersonically expanding planar plasma. For both origin and hot bands accurate spectroscopic parameters are derived from a precise rotational analysis. The origin band measurements extend earlier work and the 11(1)(1) μ(2)Σ-μ(2)Σ vibronic hot bands are discussed here for the first time. The Renner-Teller effect for the lowest bending mode ν(11) is analyzed, yielding the Renner parameters ε(11), vibrational frequencies ω(11), and the true spin-orbit coupling constants A(SO) for both (2)Π electronic states. From the Renner-Teller analysis and spectral intensity measurements as a function of plasma jet temperature, the excitation energy of the lowest-lying 11(1) μ(2)Σ vibronic state of C(6)H is determined to be (11.0 ± 0.8) cm(-1).

Excited-state lifetime of adenine near the first electronic band origin

The excited-state lifetime of supersonically cooled adenine was measured in the gas phase by femtosecond pump-probe transient ionization as a function of excitation energy between 36 100 and 37 500cm(-1). The excited-state lifetime of adenine is ∼2ps around the 0-0 band of the (1)L(b) ππ(∗) state (36 105cm(-1)). The lifetime drops to ∼1ps when adenine is excited to the (1)L(a) ππ(∗) state with the pump energy at 36 800cm(-1) and above. The excited-state lifetimes of (1)L(a) and (1)L(b) ππ(∗) states are differentiated in accordance with previous frequency-resolved and computational studies.

Re-examination of the rotationally resolved spectra of the electronic origin and several vibronic bands of 1,3-benzodioxole

The structure and vibrations of 1,3-benzodioxole are reinterpreted using rotational constants and inertial defects obtained from high resolution electronic spectroscopy. Results are examined in light of previously published theoretical calculations and conclusions are drawn regarding vibrational assignments for transitions to the excited state. Compared to the ground state, the excited S 1 state has a significantly more nonplanar geometry (and/or is considerably less rigid), owing to an enhanced anomeric effect involving the oxygen lone pair electrons.

Lifetime Broadening in the Rotationally Resolved Electronic Spectra of Dibenzothiophene, 2,5-Diphenylfuran, and 2,5-Diphenyl-1,3,4-oxadiazole in the Gas Phase. Intersystem Crossing Dynamics in the Statistical Limit.

The fluorescence lifetime of the zero point vibrational level of the first excited electronic state of dibenzothiophene (DBT) has been determined to be 1.0 ns by analysis of its rotationally resolved S1 <-- S0 fluorescence excitation spectrum. The S1 lifetime of DBT is substantially shorter than those observed for fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF), analogs of DBT in which the heavy sulfur atom is replaced by lighter ones. The electronic origin bands through the series CAR, FLU, DBF, and DBT exhibit a monotonic increase in Lorentzian broadening in their Voigt line shape profiles. Two other heterocyclic molecules manifest similar photophysical properties; 2,5-diphenylfuran and 2,5-diphenyl-1,3,4-oxadiazole. Lorentzian line shape broadenings of approximately 76 MHz were observed in the high-resolution spectra of their origin bands. Possible reasons for the short fluorescence lifetimes of these heterocycles are discussed.