scholarly journals The Populations of H2 Vibrational States in Intense UV Fields Near O-B Stars

1987 ◽  
Vol 115 ◽  
pp. 179-180
Author(s):  
P. E. Dewdney ◽  
R. S. Roger ◽  
N. Robert

In most places where molecular hydrogen exists in the interstellar medium, it will be found in the ground vibrational and ground electronic state. This will not be so, however, near 0 or early B stars where, in the region just beyond the ionization boundary, populations will be determined by UV fields up to 105 times more intense than the mean interstellar value (4 × 10−16 ergs cm−3 s−1 = 1 Habing unit). The H2 absorbs Lyman-Werner band photons longwards of λ91 nm and subsequent decays to the ground electronic state may lead to dissociation (vibrational continuum) or to one of 14 vibrationally excited states. Molecules in these states have lifetimes of order 1010 s and, in the intense fields, will be exposed to further Lyman-Werner excitation. The probability of dissociation is therefore greatly enhanced by this ‘multiple excitation’, since the number of lines available to vibrationally excited H2 is many times that available to ground-state H2 (Shull, 1978).

2020 ◽  
Vol 641 ◽  
pp. A160 ◽  
Author(s):  
M. Melosso ◽  
A. Belloche ◽  
M.-A. Martin-Drumel ◽  
O. Pirali ◽  
F. Tamassia ◽  
...  

Context. Aminoacetonitrile, a molecule detected in the interstellar medium only toward the star-forming region Sagittarius B2 (Sgr B2), is considered an important prebiotic species; in particular, it is a possible precursor of the simplest amino acid glycine. To date, observations have been limited to ground state emission lines, whereas transitions from within vibrationally excited states remained undetected. Aims. We wanted to accurately determine the energies of the low-lying vibrational states of aminoacetonitrile, which are expected to be populated in Sgr B2(N1), the main hot core of Sgr B2(N). This step is fundamental in order to properly evaluate the vibration-rotation partition function of aminoacetonitrile as well as the line strengths of the rotational transitions of its vibrationally excited states. This is necessary to derive accurate column densities and secure the identification of these transitions in astronomical spectra. Methods. The far-infrared ro-vibrational spectrum of aminoacetonitrile has been recorded in absorption against a synchrotron source of continuum emission. Three bands, corresponding to the lowest vibrational modes of aminoacetonitrile, were observed in the frequency region below 500 cm−1. The combined analysis of ro-vibrational and pure rotational data allowed us to prepare new spectral line catalogs for all the states under investigation. We used the imaging spectral line survey ReMoCA performed with ALMA to search for vibrationally excited aminoacetonitrile toward Sgr B2(N1). The astronomical spectra were analyzed under the local thermodynamic equilibrium (LTE) approximation. Results. Almost 11 000 lines have been assigned during the analysis of the laboratory spectrum of aminoacetonitrile, thanks to which the vibrational energies of the v11 = 1, v18 = 1, and v17 = 1 states have been determined. The whole dataset, which includes high J and Ka transitions, is well reproduced within the experimental accuracy. Reliable spectral predictions of pure rotational lines can now be produced up to the THz region. On the basis of these spectroscopic predictions, we report the interstellar detection of aminoacetonitrile in its v11 = 1 and v18 = 1 vibrational states toward Sgr B2(N1) in addition to emission from its vibrational ground state. The intensities of the identified v11 = 1 and v18 = 1 lines are consistent with the detected v = 0 lines under LTE at a temperature of 200 K for an aminoacetonitrile column density of 1.1 × 1017 cm−2. Conclusions. This work shows the strong interplay between laboratory spectroscopy exploiting (sub)millimeter and synchrotron-based far-infrared techniques, and observational spectral surveys to detect complex organic molecules in space and quantify their abundances.


2019 ◽  
Author(s):  
Matthew M. Brister ◽  
Carlos Crespo-Hernández

<p></p><p> Damage to RNA from ultraviolet radiation induce chemical modifications to the nucleobases. Unraveling the excited states involved in these reactions is essential, but investigations aimed at understanding the electronic-energy relaxation pathways of the RNA nucleotide uridine 5’-monophosphate (UMP) have not received enough attention. In this Letter, the excited-state dynamics of UMP is investigated in aqueous solution. Excitation at 267 nm results in a trifurcation event that leads to the simultaneous population of the vibrationally-excited ground state, a longlived <sup>1</sup>n<sub>O</sub>π* state, and a receiver triplet state within 200 fs. The receiver state internally convert to the long-lived <sup>3</sup>ππ* state in an ultrafast time scale. The results elucidate the electronic relaxation pathways and clarify earlier transient absorption experiments performed for uracil derivatives in solution. This mechanistic information is important because long-lived nπ* and ππ* excited states of both singlet and triplet multiplicities are thought to lead to the formation of harmful photoproducts.</p><p></p>


1985 ◽  
Vol 40 (9) ◽  
pp. 913-919
Author(s):  
Juan Carlos López ◽  
José L. Alonso

Abstract The rotational transitions of 3,4-dihydro-1,2-pyran in the ground state and six vibrationally excited states have been assigned. The rotational constants for the ground state (A = 5198.1847(24), B = 4747.8716(24) and C = 2710.9161(24) have been derived by fitting μa, μb and μc-type transitions. The dipole moment was determined from Stark displacement measurements to be 1.400(8) D with its principal axis components |μa| =1.240(2), |μb| = 0.588(10) and |μc| = 0.278(8) D. A model calculation to reproduce the ground state rotational constants indicates that the data are consistent with a twisted ring conformation. The average intensity ratio gives vibrational separations between the ground and excited states of the ring-bending and ring-twisting modes of ~ 178 and ~ 277 cm-1 respectively.


2015 ◽  
Vol 17 (44) ◽  
pp. 29518-29530 ◽  
Author(s):  
Matthieu Sala ◽  
Stéphane Guérin ◽  
Fabien Gatti

We propose a new mechanism for the radiationless decay of photoexcited pyrazine to its ground electronic state involving a conical intersection between the dark Au(nπ) state and the ground state.


1986 ◽  
Vol 41 (3) ◽  
pp. 483-490 ◽  
Author(s):  
O. L. Stiefvater

The earlier prediction of the preferred and the less stable rotameric conformations of isobutyraldehyde, (CH3)2CHCHO, has been confirmed experimentally by microwave spectroscopy. The compound exists mainly in a gauche conformation, in which one of the methyl groups is eclipsed by the oxygen atom, and the less stable rotamer is the trans conformation, in which the oxygen atom eclipses the isopropyl hydrogen.Ground state rotational constants (in MHz) and centrifugal distortion constants (in kHz), together with dipole moments (in D), are:Rotation spectra due to three torsionally excited states of each rotamer have been identified, along with satellites arising from CH3 internal rotation and CC2 wagging.


2019 ◽  
Vol 5 (9) ◽  
pp. eaax6625 ◽  
Author(s):  
Jennifer M. Ruddock ◽  
Haiwang Yong ◽  
Brian Stankus ◽  
Wenpeng Du ◽  
Nathan Goff ◽  
...  

We explore the photo-induced kinetics of 1,3-cyclohexadiene upon excitation at 200 nm to the 3p state by ultrafast time-resolved, gas-phase x-ray scattering using the Linac Coherent Light Source. Analysis of the scattering anisotropy reveals that the excitation leads to the 3px and 3py Rydberg electronic states, which relax to the ground state with a time constant of 208 ± 11 fs. In contrast to the well-studied 266 nm excitation, at 200 nm the majority of the molecules (76 ± 3%) relax to vibrationally hot cyclohexadiene in the ground electronic state. A subsequent reaction on the ground electronic state surface leads from the hot cyclohexadiene to 1,3,5-hexatriene, with rates for the forward and backward reactions of 174 ± 13 and 355 ± 45 ps, respectively. The scattering pattern of the final hexatriene product reveals a thermal distribution of rotamers about the carbon-carbon single bonds.


1964 ◽  
Vol 42 (10) ◽  
pp. 2176-2192 ◽  
Author(s):  
F. D. Findlay ◽  
J. C. Polanyi

When atomic plus molecular hydrogen coming from a Wood's discharge tube are mixed with molecular chlorine, infrared emission is observed (1). At low reagent pressures, ~10−2 mm Hg, this emission can be related to the relative rate of the reaction H + Cl2 → HCl†ν + Cl proceeding to form HCl in vibrationally excited states ν = 1–6, of the ground electronic state. In the present work this system has been investigated for the first time at ~100 × the reagent pressure (~1 mm Hg). The reaction was shown to proceed by a chain mechanism. The translational–rotational temperature was 1300 ± 100 °K under the experimental conditions normally used. The vibrational distribution was notable for the presence of vibrators in levels ν = 7 and 8, which are respectively 4 and 10 kcal higher in energy than the exothermicity of the H + Cl2 reaction. The population in these levels appeared to be related to that in the levels with [Formula: see text]; it was proposed that vibrational–vibrational exchange among these lower levels was responsible for populating the higher ones. A simple model yielded a collision efficiency for HCl†ν=1 + HCl†ν=6 → HCl†ν=7 + HCl†ν=0, of Z1,6t = 6 × 103 collisions per transfer. Addition of HCl to the reaction mixture brought about a redistribution among vibrationally excited states indicative of a fast vibrational transfer, HClν=0 + HCl†ν=2 → 2 HCl†ν=1.At reduced pressure of HCl† the stationary-state distribution among higher vibrational states approximated closely to that observed at 10−2 mm Hg total pressure (where collisional deactivation is insignificant), suggesting that collisional deactivation was not of major importance even at the pressure used in the present work. In order to account for the high translational–rotational temperature, in the absence of substantial vibrational deactivation, it was necessary to suppose that the greater part of the energy liberated by the reaction H + Cl2 went directly into translational and rotational motion of the products.


2009 ◽  
Vol 6 (s1) ◽  
pp. S259-S279 ◽  
Author(s):  
Masoud Motamedi ◽  
Najmehalsadat Khademi

The millimeter-wave rotational spectra of the ground and excited vibrational states v(A), v1(E) =1 and v2(E ) =1 of the oblate symmetric top molecule, (CH2O)3, have been analyzed again. The B0= 5273.25747MHz, DJ= 1.334547 kHz, DJk= -2.0206 kHz, HJ(-1.01 mHz), HJK(-3.80 mHz), and HKJ(4.1 mHz) have been determined for ground state. For non degenerate excited state, vA(1), the B = 5260.227723 MHz and DJand DJKwere determined 1.27171 kHz and -1.8789 kHz respectively. The 1=±1 series have been assigned in two different excited states v1(E) =1 and v2(E) =1.Most of the parameters were determined with higher accuracy compare with before. For the v2(E) =1 state the Cζ=-1940.54(11) MHz and qJ= 0.0753 (97) kHz were determined for the first time.


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