Infrared multiphoton excitation and dissociation studies of SO
2S. M. A. DURRANIand M. AHMED
Energy Research Laboratory, Research Institute, King Fahd University of Petroleum and Minerals Dhahran 31261, Saudi Arabia
(ricevuto il 10 Luglio 1996; approvato il 12 Giugno 1997)
Summary. — Infrared multiphoton absorption of 9R(26) and 9R(32) CO2laser lines
by SO2molecule was studied for the unfocused laser energies of 100 mJ to 253 mJ.
Dispersed fluorescence spectra extending from 3200 Å to 4500 Å confirmed the very strong coupling between the ground and excited electronic states of SO2. The shapes
of the spectra were completely different for the 9R(26) line or the neighboring 9R(32) line corresponding to the wavelengths 1082.3 cm21 and 1085.8 cm21, respectively,
and for different laser energies. This indicates that the infrared multiphoton excitation process is controlled both by the laser intensity and the wavelength. The formation of sulfur from possible dissociation or fragmentation of SO2molecules was
monitored using glass substrates suspended inside the absorption cell. Proton-Induced X-ray Emission (PIXE) measurements of these substrates show the presence of sulfur only for the 9R(32) laser line and energy of 253 mJ, providing evidence of fragmentation of SO2under these conditions.
PACS 33.20 – Molecular spectra. PACS 33.20.Ea – Infrared spectra.
1. – Introduction
In recent years the study of infrared multiphoton excitation (IRMPE) of small polyatomic molecules such as SO2has received considerable attention [1-6]. In SO2 the excited electronic states that lie below the dissociation threshold are well coupled to the high-lying vibrational levels of the ground-state electronic manifold [7]. Molecules excited to these high-lying vibronic levels through IRMPE can be observed directly by fluorescence. The fluorescence spectrum of SO2 is extremely complicated but consists of a broad structured feature extending from about 240 nm into the visible part of the spectrum [8, 9]. This molecule is representative of small-size systems which, according to the quasicontinuum model [10], can be difficult to excite to high vibrational levels since the density of states at low excitation is relatively small. While observation of inverse electronic excitation in SO2 clearly demonstrated that infrared multiphoton excitation is possible and the excitation process is controlled mainly by the laser intensity [1], however it was not clear from these studies through which vibrational mode(s) the excitation proceeds. Until recently it has been shown that only the n1mode
to the laser wavelengths of 1085.8 cm21 and 1079.8 cm21, respectively [11, 12]. In this paper we report the results on the infrared multiphoton excitation (IRMPE) and infrared multiphoton dissociation (IRMPD) of the SO2molecule and its dependence on the laser intensity and wavelength using 9R(32) and 9R(26) CO2laser lines.
2. – Experimental
We have combined two different experimental set-ups described in detail elsewhere [12-15] after some modification to detect the infrared multiphoton excitation and dissociation of selected IR radiation by SO2. In brief a high peak power Lumonics TEA CO2laser system was used to observe the excitation and dissociation of the SO2 molecule. This system produces TEM00 single longitudinal (SLM) pulses of different wavelengths that are subsequently amplified by two amplifiers. The amplified output laser beam from the amplifier is reflected on several high and partial reflectors before entering into the absorption cell. This allows sending the beam to different devices used to monitor the laser mode, wavelength and energy precisely. A 15 cm focal length ZnSe lens was used to focus the laser into the cell. The absorption cell is basically a 30 cm long and 5 cm diameter Pyrex tube with ZnSe windows on both sides. To observe the dispersed fluorescence resulting from the infrared multiphoton excitation and dissociation, an additional 2.5 cm diameter port with sapphire window at the center of the cell was also made. Moreover at the center of the tube above the focal point of the ZnSe lens (i.e. 2.5 cm diameter window), a 1 cm 3 1 cm glass substrate was suspended to collect the sulfur resulting from the possible dissociation of the SO2 molecule for each measurement reported here, i.e. 9R(26) and 9R(32) laser lines and energies from 130 mJ to 253 mJ.
Dispersed fluorescence following infrared multiphoton excitation (IRMPE) and infrared multiphoton dissociation (IRMPD) was collected by a 0.5 m spectrometer through a lens. With 500 mm entrance and exit slits, the spectral resolution of the monochromator was better than 1 A. The output of the photomultiplier tube is fed into a boxcar integrator (EG&G Model 4422) with a model 4402 signal processor from the same company. The data were collected by a computer connected through GPIB connection to the boxcar.
To detect the sulfur trace resulting from possible dissociation of the SO2molecules, a 1 cm2 glass substrate was suspended 0.5 cm above the focal point of the lens. The substrates were analyzed using the scanning microbeam PIXE facility in this laboratory [16]. A focused 2.5 MeV proton microbeam of about 5 mm resolution was used to scan a 0.5 mm 3 0.5 mm area on the sample surface to generate characteristic X-rays from the atoms present in the sample. The X-rays produced were detected by a Si (Li) detector placed at 135 degrees with respect to the incident-beam direction. The detector signals were processed by the data acquisition system to produce simultaneously an X-ray energy spectrum averaged over the scanned area as well as Si and S distribution maps. To avoid charge build-up due to proton bombardment, samples prepared on a glass substrate were coated with a thin layer of carbon to make the surface electrically conducting. A blank glass substrate from the same batch as the sample substrates was analyzed as control. Finally for all the measurements reported here the cell temperatures and pressures were 22 7C and 500 torr, respectively.
3. – Results and discussion
Figures 1a-1c and 2a-2c show the observed dispersed fluorescence emission spectrum between 3000 Å and 4500 Å after multiphoton absorption for different CO2 laser lines and energies. Figures 1a-1c are produced by excitation with 9R(26) laser line and laser energies of 130 mJ, 196 mJ and 240 mJ, respectively, while figs. 2a-2c are produced by excitation with 9R(32) laser line and laser energies of 135 mJ, 180 mJ and 253 mJ, respectively. It is clear from figs. 1 and 2 that for a specific laser line when the energy is increased from 130 mJ to 253 mJ (fig. 1a-c, fig. 2a-c) for the
Fig. 1. – SO2dispersed fluorescence of multiphoton excitation for a cell pressure of 500 torr and
Fig. 2. – SO2dispersed fluorescence of multiphoton excitation for a cell pressure of 500 torr and
9R(32) laser line (i.e. 1085.8 cm21), for (a) 133 mJ, (b) 190 mJ and (c) 253 mJ.
collected spectrum between 3200 Å and 4500 Å, the structures of different bands are manifest. In addition, more transitions have also appeared in the spectra.
Micro-PIXE measurements of the glass substrates used to monitor SO2dissociation indicated the presence of sulfur only in case of 9R(32) line and laser energy of 253 mJ (conditions corresponding to fig. 2c). Figures 3a and 3b show X-ray energy spectra from a blank glass substrate and the sample, respectively. A number of elements including a trace amount of S could be detected in the glass backing (fig. 3a). However, the sample spectrum in fig. 3b, corresponding to the broad spectrum of fig. 2c, shows the presence of a much larger amount of S arising from SO2 dissociation. Si and S
Fig. 3. – Proton-induced X-ray energy spectra from (a) blank glass substrate and (b) sample, corresponding to a cell pressure of 500 torr, 9R(32) laser line (i.e. 1085.8 cm21) and laser energy of
253 mJ.
distribution maps in fig. 4 show that the trace amount of S present in the blank is well dispersed in the Si matrix. However, S resulting from the fragmentation of SO2 deposits on the glass surface masking the Si matrix and there by attenuating the number of Si X-rays from the backing. There is also some agglomeration of S in several areas of the glass substrate.
Since for the conditions of figs. 1a-1c and 2a-2b no evidence of sulfur was seen on the substrates, possible fragmentation of SO2into S and O2can be ruled out. This could
Fig. 4. – Si and S distribution maps, each 0.5 mm2in size, for the blank glass substrate (top) and
the S deposited glass substrate (bottom). Darker shades indicate higher concentrations.
be due to the fact that the number of photons absorbed in these cases may not be sufficient to take the SO2 molecule to the threshold leading to its complete fragmentation (dissociation) into S and O2. However, the possibility of SO2 and SO emission spectra overlapping in the region 3900 Å to 3200 Å [17] cannot be ruled out. Therefore for these conditions, due to several photon absorption, the partial fragmentation (dissociation) of SO2 into SO and O is possible. Although we have no direct evidence for this, several combinations of electronic states are energetically accessible [18]. Thus the features of the emission spectra observed between 3200 Å and 4500 Å (figs. 1a-1c and 2a-2b) could be due to the result of excitation and de-excitation of the SO and/or SO2 molecule. Several transitions can contribute to these emission spectra [19]. Moreover we believe that the broad spectra of fig. 2c, for which the fragmentation of SO2 into S and O2 has been observed (fig. 3b), could be due to the excitation and de-excitation of SO, SO2, S2 and O2. Since, apart from SO and SO2, the contribution of strong emission both from S2 and O2 in the region between 3200 Å to
4500 Å could not be ignored. Hence in this case several transitions can con-tribute [9, 18, 19].
The present work shows that the 9R(32) line at 253 mJ of energy is suitable for an efficient multiphoton excitation of SO2leading to the fragmentation of this molecule.
* * *
This work is part of the ERL project supported by the Research Institute of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia.
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