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STUDY OF HIGHLY EXCITED VIBRATIONAL STATES OF CHD3, CHF3, SiH4, SiH2Cl2 BY
INTRACAVITY LASER ABSORPTION SPECTROSCOPY (ICLAS)
A. Campargue, H. Ben Kraiem, M. Chenevier, F. Stoeckel
To cite this version:
A. Campargue, H. Ben Kraiem, M. Chenevier, F. Stoeckel. STUDY OF HIGHLY EXCITED VI- BRATIONAL STATES OF CHD3, CHF3, SiH4, SiH2Cl2 BY INTRACAVITY LASER ABSORP- TION SPECTROSCOPY (ICLAS). Journal de Physique Colloques, 1987, 48 (C7), pp.C7-743-C7-745.
�10.1051/jphyscol:19877183�. �jpa-00227008�
JOURNAL DE PHYSIQUE
Colloque C7, suppldment au n012, Tome 48, decembre 1987
STUDY OF HIGHLY EXCITED VIBRATIONAL STATES OF CHD,, CHF,, SiH,, SiH,Cl, BY INTRACAVITY LASER ABSORPTION SPECTROSCOPY (ICLAS)
A. CAMPARGUE, H. BEN KRAIEM, M. CHENEVIER and F. STOECKEL
Laboratoire de Spectrom6trie Physique (CNRS UA-08 et Celphyra), Universit6 Scientifique, Technologique et Mgdicale de Grenoble, BP 87. F-38402 Saint-Martin-d'HBres Cedex, France
ABSTRACT
By using a visible and an IR ICLAS spectrometer coupled with a high resolution spectrograph ( X / a X = 800000) we have studied the high vibrational excited states in the ground state of some small polyatomic molecules. The study was focussed on the transition involving several quanta of stretching and bending of one to four X-H bonds, with X=C,Si. Rotational constants including B', C ' , D'j, D'jr and D k ' are given, perturbation in frequency and coupling between stretching and bending of large amplitude are discussed.
I. INTRODUCTION.
The study of high vibrational excited states in the ground state of polyatomic molecules is of great interest in the study of selective photochemistry and in multiphoton excitation processes. Unfortunately, the transitions between the ground vibrational level and these highly excited states are very weak and mostly rotationaly unresolved. Small electric and potential anharmonicity is responsible for the weakness of the transitions and the moment of inertia is the cause of the small rotational constants.
In this paper we will show that it is possible to overcome these difficulties by using a highly sensitive, quantitative and high resolution intracavity laser spectroscopy technique. The understanding of the internal vibrational relaxation mechanism could be given by the coupling between the vibrations of the bonds which is detected by the vibrational line positions, while the rotational line widths gives information about the localisation of the energy in one bond.
Until now FTIR spectroscopy with a long absorption path length has been used for transition in the near infrared, while the photoacoustic technique was used in the visible range. Here we use the ICLAS technique, has been developed a few year ago in our laboratory.
11. EXPERIMENTAL
Spectra were recorded with two different ICLAS spectrometers. The first was used to record overtone transitions in the visible range (ICLAS-VIS), the second one in the infrared region (ICLAS-IR). While the optical detection is slightly different for the two spectrometers,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19877183
JOURNAL DE PHYSIQUE
the principle is the same. The technique is based on the observation of the time resolved spectrum of the beam emitted by the laser when weakly absorbing species are placed inside the cavity [I]. The linewidths of the absorbing species, larger than the mode spacing of the laser must be narrower than the linewidth of the laser. The time tg spent from the starting of the laser to the observation of the spectrum, gives directly the equivalent pathlength lcq = (R/L).c.tp, where c is the velocity of light, R the length of the absorbing cell, and L the length of the cavity of the laser. An equivalent pathlength of a few hundreds of kilometers can be obtained.
For the ICLAS-VIS spectrometer, the ,,o,,,, ',,o" ,,,,,,,,",,,",,,,,H
laser is a cw dye laser pumped by an Art laser. The beam is send into a high resolution grating spectrograph (nX/X = 800000, ao = 0.02 cm-1 at 16000 cm-I ) developped in collaboration with the SOPRA compagny. The spectrum is recorded with a photodiode array and the timing is fixed by means of two acousto optic modulators.
The figure opposite shows the experimental setup of the ICLAS-VIS spectometer
For the ICLAS-IR spectrometer [ 2 ] , the laser is a color center laser pumped by a Nd:Yag, the grating spectrograph has a lower resolution and is scanned, and the spectrum is recorded with a cooled Ge detector. The timing is given by the pulsed Nd:Yag laser and the boxcar.
CEDI and CHFa C31
1
The spectrum of CHD3 is recorded near 13500 cm-I for the polyad N=5 and near 16100 cm-I for N=6, where N = vs + vb 12, vs is the quantum number of the C-H stretching mode
vi (A1 ) and vb the quantum number of H-C-D bending mode US (El. Six bands were observed with a pressure varying in the range of a few tens to a few hundreds of torr and the equivalent pathlength between a few kilometers and some tens of kilometer.
The figure opposite shows as an example , part of the P branch of the band N=6 (vs.=5,vb=2). The spectral is resolution 0.02cm-',pressure 476 Torr, equivalent pathlength 13.5 kilometer.
Each rotational structure was well resolved and identified for the small bending quantum numbers , while the spectra was much perturbated and individual lines only partially resolved for higher bending quantum number.
This polyad was analysed using the effective hamiltonian of M. Quack et al. The variation of the rotational constant B" and C" can be explained in part by an effective vibrational potential interaction
Table I. Rotational constant (cm-I> of CHDs
vs vb Wavenumber B' -B" ~ 1 - c ~ DJ ' -DJ " DJ K ' -DJ K " DK ' -DK "
x106 x106 x106 2,6> 13192.610
3,4> 13404.73(2) -0.0862(1) 31.5(8.0)
4,2> 13666.142(8) -0.0272(3) 0.0396(3) -22.6(2.9) -13(6) 5 5(4) 5,0> 13799.454(4) -0.0747(9) -0.0441(1) -7.4(4) 8.9(9) -3.8(8) 3 , 6 > 15933.80 (2) -0.10611) -0.3007115)
For CHF3 ,five band were observed between 13400 and 14400 cm-1 for the polyad N=5. In contrast with CHD3, at a pressure of 104 Torr and with a resolution of 0.02 cm-1, no J or K rotational structure was found, but only band contours characteristic of the parallel At-AX bands of symmetric top molecules. Table I1 gives the observed and calculated energy and intensity for each band of the polyad N=5. Generally, good agreement was obtained for both the energies and intensities.
SiHsClz and SiHl [21
For SiHzClz, a local mode model of two coupled anharmonic oscillators was used, to describe the observed bands. Two unresolved bands corresponding of the av(Si-H) at 6500 cm-I and one at 14163.9 cm-I (av(Si-H) = 7 were observed. Diagonal . anharmonicity, kinetic and potential off diagonal energy coupling was deduce from our work and previous results.
For SiH4, a local mode model of Halonen and Child of four harmonically coupled Morse oscillators predicts the stretching vibrational levels.
The band at 6497.62 cm-1 is rotationally analysed and the intensity of the band measured G = (1.5
+
0.5) x lo-= (pm)=.We have shown that intracavity laser spectroscopy is very well suited for the study of the highly excited vibrational states in small polyatomic molecule. These states are only accessible by weak transitions from the ground state and gives a dense rotational spectrum. For these reason the high sensitive and high resolution ICLAS spectroscopic technique is well suited for this kind of study.
References:
[I] Intensity measurements and self broadening coefficients in the gamma band of 02 at 628 nm using intracavity laser absorption spectroscopy (ICLAS)
M.A. MELIERES, M. CHENEVIER, F. STOECKEL
J. Quant. Spectrosc. Radiat. Transfer 33, 1985, 337'
[2] Intracavity spectroscopy of overtone transition is SiH4 SiHzClz and SiHC13
A-CAMPARGUE, F-STOECKEL, M.C. TERRILE Chem. Phys. 110, 1986, 145 131 Highly excited vibrational states of CHF3 and CHD3 in the range of the vs = 5 CIi chromophore
A. CAMPARGUE and F. STOECKEL J. Chem. Phys. Vol. 85, n o 3, 1220, 1986
[ 4 1 Spectroscopy and dynamics of the isolated CiI chromophore in CDIH :
Experiment and theory.
S. PEYERIMHOFF, M. LEWERENZ and M. QUACK
Chemical Physics Letters, Vol. 109, no 6 , 563, 1984
