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Laser-induced fluorescence of selectively excited bands located within the E band's spectral range of SO2
15 April 1994
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 221 (1994) 33-38
Laser-induced fluorescence of selectively excited bands located within the E band’s spectral range of SO2 E. Hegazi, A. Hamdan, F. Al-Adel Laser Research Laboratory, King Fahd University ofPetroleum andMinerals, Dhahran 31261, Saudi Arabia Received 3 August 1993; in final form 11 February 1994
Abstract An attempt to investigate the vibrational bands near the origin of the E band of SO* has been made in a supersonic jet experiment. Several cold vibrational bands have been identified within the 32850-32880 cm-’ region and their fluorescence spectra have been selectively excited and measured. A hot band activity at * 32872 cm-’ was found to persist at low rotational temperatures, which caused the Franck-Condon intensity pattern of the E band’s fluorescence spectrum to vary when measured under slightly different cooling conditions. The band’s upper levels were not all found to have the same vibronic symmetry as that of the E band.
1. Introduction
Ab initio calculations performed on the SO2 molecule [ 1,2] predict two electronic states: ‘Br and ‘AZ, whose energies lie within the 2400-3600 A spectral range from the ‘Al ground state. The experimentally measured absorption spectra in this region are known to show weak transitions at the long-wavelength end and more intense ones suddenly appearing below 3200 A. According to the measured isotope shifts of several bands [3] above and below 3200 A, all of these transitions appeared to originate from only one upper electronic state. The rotational analysis performed on several of these bands revealed [ 4,5 ] further that this upper electronic state was not the ‘B1 state but the ‘A2 state, from which transitions into the ‘A, ground state should be electronically forbidden. A possible explanation for the presence of these transitions must then be based on efficient vibronic coupling between the ‘Br and ‘AZ states through the unsymmetric mode of vibration. This vibronic interaction, along with possible spin-orbit coupling of
these two states with the lowest triplet states, are believed to be the main cause for the severe perturbation in the rotational structure [ 61 of the Clement’s A, B, C, ... bands [ 7,8] throughout the whole spectral region. Out of these letter bands, the least perturbed one was found to be the E band, and that made it a prime target for rotational analysis. Shaw et al. [9] recorded extensive fluorescence spectra of excited single vibronic levels of the 34002600 A region and interpreted the vibrational structure in terms of ‘B1 levels spreading among the ‘AZ levels. For the E and G bands, in particular, they reported a change in the rotational structure as the excitation was moved across each band, concluding that these two bands should be described as linear combinations of many vibronic levels rather than simple products of electronic and vibrational wavefunctions. By using a supersonic beam in which SOz was seeded in Ar at a low [SO,]/ [Ar] value of 0.03, Kullmer and Demtroder [ 10,111were able to produce rotational spectra in which only the lowest rotational levels were populated ( T,,, = 10 K) and to
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved .SsDI0009-2614(94)00219-G
34
E. Hegazi et al. / Chemical Physics Letters 221(1994) 33-38
deduce the molecular constants for the E band’s upper level. The analysis of their spectra revealed that, near the E band’s upper level, there must exist several upper vibrational levels within a narrow energy range of a few cm-‘. Rotational lines of only less than 30% of the existing upper levels within the 32820-32885 cm-’ region could be identified, while most of the weak lines and about 20% of the strong ones are still not assigned. In the present work, we investigate the weak vibrational bands within the 32850-32880 cm-’ spectral region by measuring their selectively excited fluorescence spectra. To be able to selectively excite these bands without exciting the E band itself we had to excite the fluorescence spectra under conditions of very low rotational temperatures ( r,, < 10 K) . This was done by employing a supersonic-jet technique similar to that of Fischer et al. [ 111, in which further rotational cooling was achieved by mixing SO2 with Ar at low [SO*] / [Ar] ratio. While for the rotational analysis of ref. [ 111 a ratio of 1: 30 was used, in the present work we had to use ratios in the range between 1: 20 and as low as 1: 250 to obtain vibrational bands with as much diminished rotational overlap as possible. The effect of changing the mixing ratio (i.e. the concentration) of the seeded SO2 on the rotational and vibrational temperatures was reported in a previous work [ 12 1, where it was evident that low SOz concentration in argon corresponded to colder rotational temperatures, while the converse was true for vibrational temperatures.
molecular beam at selected laser-to-nozzle distances (5-20 mm), and the resulting fluorescence signal was detected by a photomultiplier at 90” to the plane of the laser and molecular beam. The signal integrating and averaging system consisted of an EG&G boxcar model 4402/4422 and an IBM AT microcomputer, which also controlled the stepper motor of the dye laser. The excitation spectra were recorded by scanning the dye laser while detecting the total fluorescence signal through a Melles Griot GG395 filter and onto a photomultiplier. The fluorescence spectra, on the other hand, were recorded by setting the dye laser at particular exciting wavelengths while scanning the monochromator throughout the appropriate spectral range. With 350 pm entrance and exit slits, the spectral resolution of the monochromator was better than 0.5 A, which corresponded to 5 cm-’ at 3200 A. The dye used in the experiment was sulforhodamine-640 in ethanol (6090-6010 A), which after frequency doubling its output, covered the spectral range near the E band’s origin. No rotational analysis of the band contours was made, therefore only rough values of the cold rotational temperatures were estimated in terms of the SO2 concentration in the carrier gas [ 12 1. According to Kullmer and Demtroder [ 13 1, a concentration of 3% corresponded to T,, = 10 K. Hence, at the lowest ( -C1%) and highest ( 10%) concentrations used in our experiment, the coldest and hottest rotational temperatures were assumed to be considerably less than 10 K and larger than 10 K.
2. Experimental 3. Results and discussion The arrangement of the apparatus and the details of experimental procedures have been described elsewhere [ 121. SO2 gas of 99.97% purity (air Products UK) seeded in Ar was allowed to expand from a highpressure line ( x 3 atm absolute) into a vacuum chamber ( x 10-6 mbar) through a pulsed valve of 300 urn diameter nozzle. The seeding of SO* was done on-line before the expansion and through an adjustable needle valve, which controlled the SO2 concentration with respect to the carrier gas. Nanosecond UV laser pulses of 0.5 cm-’ band width, generated by frequency doubling the output of a Nd: YAGpumped dye laser, was directed perpendicular to the
Fig. 1 shows excitation spectra near the E band’s origin of jet-cooled SO1 recorded by monitoring the laser-induced total fluorescence signal. The supersonic-jet conditions were the same for all spectra except that the concentration of SO2 with respect to the carrier gas was lOoh in (a), 2% in (b), and
E. Hegazi et al. /Chemical PhysicsLetters221(1994) 33-38
32820
32830
32840
32850 32860 Wavenumber
32870 (cm-l)
32880
32890
32900
3000
3200
3400
3800
3800 Angstram
4000
4200
4400
4800
Fig. 1. Excitation spectra recorded by monitoring the total fluorescence signal. The SOz concentration was 10% for (a), 2% for (b ), and c 1% for (c ) . The vertical lines at the bottom indicate the excitation energies used to produce the fluorescence spectra in Figs. 2 and 4.
Fig. 2. Dispersed fluorescence spectra of the shifted E band (a) and the E band (b) excited at 32865 and 32872 cm-‘, respectively. Both spectra were produced with SO1 concentration of o 2%. The spectrum in (c) is of the excited E band also, but produced with SO2 concentration of z 10%.
at around 32855 cm-’ [ 111, is now limited mainly to a spectral range of less than 10 cm-’ and a maximum intensity centered more or less around the band’s origin at 32872 cm- ‘. This shrinking of the E band allowed a few vibronic bands to be seen underneath its spectral range as should be expected according to Kullmer and Demtroder [ 10 1. The peak at 32865 cm-‘, which is well resolved in Fig. lc, is the shifted E band as identified by them, while the peaks appearing around 32855 cm-’ are unidentified vibronic bands whose rotational lines are probably the ones that Kullmer and Demtroder could not assign to rotational transitions with the proper selection rules
tra of the E band (b) and the shifted E band (a) excited at 32872 cm-’ and 32865 cm-‘, respectively, with SO2 concentration of x 2W. The ‘Ai levels into which the prominent transitions from the upper levels of the E band and the shifted E band terminate are indicated on the spectra. The vibrational analysis was carried out using the anharmonic expansion of the vibrational energy,
1101. Although now the spectral range underneath this part of the E band has become of more defined vibrational features it is still not possible yet to conclusively identify any peak as corresponding to a cold or hot band. This is because of the substantial overlap occurring at high concentration, which makes the comparison of the band’s relative intensities as a function of vibrational temperature inaccurate. It is possible under such rotational cooling, however, to excite the resolved vibrational bands selectively and then compare the corresponding emitted fluorescence spectra. This we have done for several bands within the 32850-32880 cm-’ region at various rotational temperatures corresponding to concentrations between 10% and less than 1W. Fig. 2 shows the gross dispersed fluorescence spec-
G(Y~, ~2,v3)=
C
I
Wi(Vi+O.5)
+ C Xij(Vi+O*5)(Vj+O*5) iQ +
i<~~kYijk(Vi+o.5)(~j+0.5)(v*+0.5)
Y
(1)
where the various vibrational parameters of the ground state were taken from ref. [ 141. The Franck-Condon patterns of the progressions in one spectrum are apparently different from the corresponding ones in the other spectrum. For example, the nv; progression is the most intense progression in curve (b) of Fig. 2 but it is of very weak intensity in curve (a) of Fig. 2. In fact, the unexpected large intensity of the ( 100) member of this progression in curve (a) is mostly due to a background intensity of a coinciding transition into ( 100 ) in the E band system (shifted by 7 cm - ’ ). This can be illustrated in Fig. 3, which depicts three excitation spectra recorded while monitoring the fluorescence signal at the (a) 3531 A, (b) 3152 A, and (c) 3256
E. Hegazi et al. / Chemical Physics Letters 221(1994)
36
32830
32840
32850
32860 Wavenumber
32870 (cm-j)
32880
32890
32900
Fig. 3. Excitation spectra recorded while monitoring the fluorescence signal at (a) 3531 A, (b) 3152 A, and (c) 3256 A. All spectra were produced with SOz concentration of = 2%. The dotted vertical runs through the shifted E band’s origin.
8, wavelengths corresponding to transitions from the upper level of both bands into the (400), ( 100) and (200) ground state levels, respectively. In curve (a) of Fig. 3, the excitation spectrum includes both neighboring bands, which implies that the nv; progression in the shifted E band fluorescence spectrum does exist and that its transition into the (400) level at 353 1 A, although coinciding with its intense counterpart of the E band system that appear in the background, has to some extent a measurable intensity. In curve (b ) of Fig. 3, the intensity of the shifted E band is negligible compared to that of the E band indicating that the peak at 3152 8, in curve (a) of Fig. 2 corresponds mainly to the transition into ( 100) from the E band as a background intensity, and that the Franck-Condon factor for the ( 100) member in the shifted E band system is very small. Finally, in the excitation spectrum of curve (c) of Fig. 3, the E band and the shifted E band appear with equal intensity at 2% concentration indicating that, in curve (a) of Fig. 2, about 50% of the intensity of the (200) member in the (nO0) progression is due to a background intensity from the transition into the ‘A1 (200) level in the E band system. The background intensity in the above example has a dramatic contribution because the (nO0) progression in curve (b) of Fig. 2 has a large intensity. For other transitions, the background intensity in curve (a) of Fig. 2 will be considerably lower but not equal throughout the spectrum. At higher SO1 concentra-
33-38
tion upper rotational levels of the E band will be also populated and the shifted E band’s fluorescence spectrum will be mutated even further. Therefore, the relative intensities of the transitions in curve (a) of Fig. 2 will not reflect the actual Franck-Condon patterns unless the concentration of SO* is low enough so that the two bands in Fig. 1 are well separated. For the E band’s fluorescence spectrum, on the other hand, the intermingling background intensity from the neighboring shifted E band will be, in general, negligible, especially for the strong transitions. However, there is another reason why its FranckCondon patterns will change as the SO;! concentration is varied and that is the presence of a hot band very near to its origin. Curve (c) of Fig. 2 shows the E band’s fluorescence spectrum excited by the same wavelength as in curve (b) but under lower SOz concentration ( < 1%). The relative intensities of the transitions in one spectrum now appear to be slightly different form the other, indicating that another band has also been excited by the same wavelength. According to ref. [ 12 1, this latter band has to be a hot band and, in fact, it can be assigned as the one sharing the same upper vibronic level with Clement’s L band, and having the lower level as ‘A, (010). This conclusion was supported further by noticing that the intensity ratio ZE/Z&i*d E was considerably larger at high concentration values (z 10%) than at low values ( 5 I%), which is a typical hot band behavior
[121At SOz concentration of zz 3% the E band’s fluorescence spectrum was found to be identical to the one produced with 2% concentration in terms of the transitions’ relative intensities. Therefore, although with the latter concentration close vibrational bands may not be very well resolved, the background intensity due to possible hot bands in the same region will be negligible. Also, the weak bands around 32855 cm-’ are reasonably far from the intense region of the E band under the 3% concentration condition and therefore their selectively excited fluorescence spectra at this concentration will reflect more or less their actual Franck-Condon patterns. The fluorescence spectra of three relatively weak bands around 32855 cm-’ are sown in Fig. 4 along with some identified lower ‘A, levels into which they decay. The vibrational analysis was carried out using the anharmonic parameters of ref. [ 141, which were
E. Hegazi et al. / Chemical PhysicsLetters221(1994) 33-38
3000
3200
3400
3600
3600
4000
4200
4400
4600
Angstrem
Fig. 4. Fluorescence spectra of excited weak bands at (a) 32850 cm-r, (b) 32853 cm-t, and (c) 32856 cm-‘. All spectra were produced with =3% concentration. The numbers identify the ‘A, (n, m, I) vibrational level into which transitions terminate.
32620
32630
32640
32850 32660 Wavenumber
32670 (cm-l)
32660
32690
32900
Fig. 5. Excitation spectra produced while monitoring the fluorescence signal at (a) 3721 A, (b) 3720 A, and (c) 3719 A. The SO1 concentration was 2% for all spectra. The fluorescence spectra in Fig. 4 were produced by exciting the marked bands in (b).
in fact useful in identifying the band origins to within f 1 cm-’ accuracy. For the fluorescence spectra of Fig. 4, those origins were determined as: (a) 32850 cm-‘, (b) 32853 cm-‘, and (c) 32856 cm-‘, and their corresponding bands can be seen separated from each other in curve (b) of Fig. 5, which depicts the excitation spectrum recorded while monitoring the fluorescence transition at 3720 A. The latter wavelength corresponds to a weak transition in the E band system and, at the same time, lies near strong transitions in the other bands’ systems. Similar excitation spectra are shown in curves (a) and (c) of Fig. 5 where the monochromator was set at 372 1 and 37 19
31
A, respectively. The change in the relative intensities in Fig. 5 is apparently occurring for cold vibrational bands in this region and not for the E band’s rotational structure. Excitation spectra similar to the ones shown in Fig. 5 could be produced such that every single vibrational band can be singled out from its neighbors, regardless of the resolution limit, by monitoring the proper fluorescence wavelength. We have compared several of these excitation spectra and have identified the following bands to be relatively the most intense ones: 32848, 32851, 32853, 32855, 32868, 32871,32877 and 32879 cm-’ ? 1 cm-’ in addition to the E band and the shifted E band. As pointed out in ref. [ lo], and seen here also, the investigated spectral range of the E band comprises of several adjacent vibrational levels that decay to the ‘AI (000) level. We find in the present work, however, that the upper levels of these bands do not all have the same vibronic symmetry. This can be seen by examining the vibrational transitions in the various resonance fluorescence spectra. For the E band fluorescence spectrum, transitions into levels having odd quanta of the unsymmetric mode v; are weak and limited only to the lowest levels such as 00 1, 10 1, 111, etc., and can be attributed to weak Coriolis coupling. The same features are also noticed for other bands such as the shifted E band and the 32856 cm-’ band in Fig. 4. Therefore, to fulfill the vibrational selection rules for the CzVpoint group, these bands’ upper levels must have either Br.a, or A2.b2 symmetry. The fluorescence spectra of the bands around 328 5 5 cm-‘, however, show relatively intense transitions into lower levels having odd quanta of v;. A typical example of such spectra is that of the excited 32856 cm-’ band, and to some extent the 32853 cm-’ band, which are shown in curves (a) and (b) of Fig. 4, respectively. These spectra show also transitions into lower levels of pure symmetric character, indicating that the vibronic symmetry of these bands’ upper levels, and possibly those of a few other bands in this region, is an admixture of Br.ar (or A,.b*) plus BI.bz (or A,.a, ) symmetries.
Acknowledgement Support from the research institute greatly appreciated.
of KFUPM
is
38
E. Hegazi et al. /Chemical PhysicsLetters221(1994) 33-38
References
[S] N. Metropolis, Phys. Rev. 60 (1941) 295.
[ 91 R. Shaw, J. Kent and M.F. O’Dwyer, J. Mol. Spectry. 82 (1980) 1.
[ lo] R. Kullmer and W. Demtroder, Chem. Phys. 81 ( 1984) [ 1 ] I.H. Hillier and V.R. Saunders, Trans. Faraday Sot. 66 (1976) 1544;Mol.Phys.22 (1971) 193. [2] D.D. Lindley, M.Sc. Thesis, Ohio State University, Columbus, OH (1976). [3] J.C.D. BrandandR.Nanes, J. Mol. Spectry. 46 (1973) 194. [4] Y. Hamada and A.J. Merer, Can. J. Phys. 52 (1974) 1443. [S] R.N. Dixon and M. Halle, Chem. Phys. Letters 22 ( 1973) 450. [6] Y. Hamada and A.J. Merer, Can. J. Phys. 53 (1975) 2555. [7] J.H. Clements, Phys. Rev. 47 (1935) 224.
2919.
[ 111 A. Fischer, R. Kullmer and W. Demtroder, Chem. Phys. 83 (1984) 415.
[ 121 F. Al-Adel, A. Hamdan, 0. Binbrek and J.S. Baskin, Chem. Phys. Letters 189 (1992) 23.
[ 131 R. Kullmer and W. Demtroder, Chem. Phys. 92 (1985) 423. [ 141 K. Yamanouchi, S. Takeuchi and S. Tsuchiya, J. Chem. Phys. 92 (1990) 4044.
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical Physics Letters 221 (1994) 33-38
Laser-induced fluorescence of selectively excited bands located within the E band’s spectral range of SO2 E. Hegazi, A. Hamdan, F. Al-Adel Laser Research Laboratory, King Fahd University ofPetroleum andMinerals, Dhahran 31261, Saudi Arabia Received 3 August 1993; in final form 11 February 1994
Abstract An attempt to investigate the vibrational bands near the origin of the E band of SO* has been made in a supersonic jet experiment. Several cold vibrational bands have been identified within the 32850-32880 cm-’ region and their fluorescence spectra have been selectively excited and measured. A hot band activity at * 32872 cm-’ was found to persist at low rotational temperatures, which caused the Franck-Condon intensity pattern of the E band’s fluorescence spectrum to vary when measured under slightly different cooling conditions. The band’s upper levels were not all found to have the same vibronic symmetry as that of the E band.
1. Introduction
Ab initio calculations performed on the SO2 molecule [ 1,2] predict two electronic states: ‘Br and ‘AZ, whose energies lie within the 2400-3600 A spectral range from the ‘Al ground state. The experimentally measured absorption spectra in this region are known to show weak transitions at the long-wavelength end and more intense ones suddenly appearing below 3200 A. According to the measured isotope shifts of several bands [3] above and below 3200 A, all of these transitions appeared to originate from only one upper electronic state. The rotational analysis performed on several of these bands revealed [ 4,5 ] further that this upper electronic state was not the ‘B1 state but the ‘A2 state, from which transitions into the ‘A, ground state should be electronically forbidden. A possible explanation for the presence of these transitions must then be based on efficient vibronic coupling between the ‘Br and ‘AZ states through the unsymmetric mode of vibration. This vibronic interaction, along with possible spin-orbit coupling of
these two states with the lowest triplet states, are believed to be the main cause for the severe perturbation in the rotational structure [ 61 of the Clement’s A, B, C, ... bands [ 7,8] throughout the whole spectral region. Out of these letter bands, the least perturbed one was found to be the E band, and that made it a prime target for rotational analysis. Shaw et al. [9] recorded extensive fluorescence spectra of excited single vibronic levels of the 34002600 A region and interpreted the vibrational structure in terms of ‘B1 levels spreading among the ‘AZ levels. For the E and G bands, in particular, they reported a change in the rotational structure as the excitation was moved across each band, concluding that these two bands should be described as linear combinations of many vibronic levels rather than simple products of electronic and vibrational wavefunctions. By using a supersonic beam in which SOz was seeded in Ar at a low [SO,]/ [Ar] value of 0.03, Kullmer and Demtroder [ 10,111were able to produce rotational spectra in which only the lowest rotational levels were populated ( T,,, = 10 K) and to
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved .SsDI0009-2614(94)00219-G
34
E. Hegazi et al. / Chemical Physics Letters 221(1994) 33-38
deduce the molecular constants for the E band’s upper level. The analysis of their spectra revealed that, near the E band’s upper level, there must exist several upper vibrational levels within a narrow energy range of a few cm-‘. Rotational lines of only less than 30% of the existing upper levels within the 32820-32885 cm-’ region could be identified, while most of the weak lines and about 20% of the strong ones are still not assigned. In the present work, we investigate the weak vibrational bands within the 32850-32880 cm-’ spectral region by measuring their selectively excited fluorescence spectra. To be able to selectively excite these bands without exciting the E band itself we had to excite the fluorescence spectra under conditions of very low rotational temperatures ( r,, < 10 K) . This was done by employing a supersonic-jet technique similar to that of Fischer et al. [ 111, in which further rotational cooling was achieved by mixing SO2 with Ar at low [SO*] / [Ar] ratio. While for the rotational analysis of ref. [ 111 a ratio of 1: 30 was used, in the present work we had to use ratios in the range between 1: 20 and as low as 1: 250 to obtain vibrational bands with as much diminished rotational overlap as possible. The effect of changing the mixing ratio (i.e. the concentration) of the seeded SO2 on the rotational and vibrational temperatures was reported in a previous work [ 12 1, where it was evident that low SOz concentration in argon corresponded to colder rotational temperatures, while the converse was true for vibrational temperatures.
molecular beam at selected laser-to-nozzle distances (5-20 mm), and the resulting fluorescence signal was detected by a photomultiplier at 90” to the plane of the laser and molecular beam. The signal integrating and averaging system consisted of an EG&G boxcar model 4402/4422 and an IBM AT microcomputer, which also controlled the stepper motor of the dye laser. The excitation spectra were recorded by scanning the dye laser while detecting the total fluorescence signal through a Melles Griot GG395 filter and onto a photomultiplier. The fluorescence spectra, on the other hand, were recorded by setting the dye laser at particular exciting wavelengths while scanning the monochromator throughout the appropriate spectral range. With 350 pm entrance and exit slits, the spectral resolution of the monochromator was better than 0.5 A, which corresponded to 5 cm-’ at 3200 A. The dye used in the experiment was sulforhodamine-640 in ethanol (6090-6010 A), which after frequency doubling its output, covered the spectral range near the E band’s origin. No rotational analysis of the band contours was made, therefore only rough values of the cold rotational temperatures were estimated in terms of the SO2 concentration in the carrier gas [ 12 1. According to Kullmer and Demtroder [ 13 1, a concentration of 3% corresponded to T,, = 10 K. Hence, at the lowest ( -C1%) and highest ( 10%) concentrations used in our experiment, the coldest and hottest rotational temperatures were assumed to be considerably less than 10 K and larger than 10 K.
2. Experimental 3. Results and discussion The arrangement of the apparatus and the details of experimental procedures have been described elsewhere [ 121. SO2 gas of 99.97% purity (air Products UK) seeded in Ar was allowed to expand from a highpressure line ( x 3 atm absolute) into a vacuum chamber ( x 10-6 mbar) through a pulsed valve of 300 urn diameter nozzle. The seeding of SO* was done on-line before the expansion and through an adjustable needle valve, which controlled the SO2 concentration with respect to the carrier gas. Nanosecond UV laser pulses of 0.5 cm-’ band width, generated by frequency doubling the output of a Nd: YAGpumped dye laser, was directed perpendicular to the
Fig. 1 shows excitation spectra near the E band’s origin of jet-cooled SO1 recorded by monitoring the laser-induced total fluorescence signal. The supersonic-jet conditions were the same for all spectra except that the concentration of SO2 with respect to the carrier gas was lOoh in (a), 2% in (b), and
E. Hegazi et al. /Chemical PhysicsLetters221(1994) 33-38
32820
32830
32840
32850 32860 Wavenumber
32870 (cm-l)
32880
32890
32900
3000
3200
3400
3800
3800 Angstram
4000
4200
4400
4800
Fig. 1. Excitation spectra recorded by monitoring the total fluorescence signal. The SOz concentration was 10% for (a), 2% for (b ), and c 1% for (c ) . The vertical lines at the bottom indicate the excitation energies used to produce the fluorescence spectra in Figs. 2 and 4.
Fig. 2. Dispersed fluorescence spectra of the shifted E band (a) and the E band (b) excited at 32865 and 32872 cm-‘, respectively. Both spectra were produced with SO1 concentration of o 2%. The spectrum in (c) is of the excited E band also, but produced with SO2 concentration of z 10%.
at around 32855 cm-’ [ 111, is now limited mainly to a spectral range of less than 10 cm-’ and a maximum intensity centered more or less around the band’s origin at 32872 cm- ‘. This shrinking of the E band allowed a few vibronic bands to be seen underneath its spectral range as should be expected according to Kullmer and Demtroder [ 10 1. The peak at 32865 cm-‘, which is well resolved in Fig. lc, is the shifted E band as identified by them, while the peaks appearing around 32855 cm-’ are unidentified vibronic bands whose rotational lines are probably the ones that Kullmer and Demtroder could not assign to rotational transitions with the proper selection rules
tra of the E band (b) and the shifted E band (a) excited at 32872 cm-’ and 32865 cm-‘, respectively, with SO2 concentration of x 2W. The ‘Ai levels into which the prominent transitions from the upper levels of the E band and the shifted E band terminate are indicated on the spectra. The vibrational analysis was carried out using the anharmonic expansion of the vibrational energy,
1101. Although now the spectral range underneath this part of the E band has become of more defined vibrational features it is still not possible yet to conclusively identify any peak as corresponding to a cold or hot band. This is because of the substantial overlap occurring at high concentration, which makes the comparison of the band’s relative intensities as a function of vibrational temperature inaccurate. It is possible under such rotational cooling, however, to excite the resolved vibrational bands selectively and then compare the corresponding emitted fluorescence spectra. This we have done for several bands within the 32850-32880 cm-’ region at various rotational temperatures corresponding to concentrations between 10% and less than 1W. Fig. 2 shows the gross dispersed fluorescence spec-
G(Y~, ~2,v3)=
C
I
Wi(Vi+O.5)
+ C Xij(Vi+O*5)(Vj+O*5) iQ +
i<~~kYijk(Vi+o.5)(~j+0.5)(v*+0.5)
Y
(1)
where the various vibrational parameters of the ground state were taken from ref. [ 141. The Franck-Condon patterns of the progressions in one spectrum are apparently different from the corresponding ones in the other spectrum. For example, the nv; progression is the most intense progression in curve (b) of Fig. 2 but it is of very weak intensity in curve (a) of Fig. 2. In fact, the unexpected large intensity of the ( 100) member of this progression in curve (a) is mostly due to a background intensity of a coinciding transition into ( 100 ) in the E band system (shifted by 7 cm - ’ ). This can be illustrated in Fig. 3, which depicts three excitation spectra recorded while monitoring the fluorescence signal at the (a) 3531 A, (b) 3152 A, and (c) 3256
E. Hegazi et al. / Chemical Physics Letters 221(1994)
36
32830
32840
32850
32860 Wavenumber
32870 (cm-j)
32880
32890
32900
Fig. 3. Excitation spectra recorded while monitoring the fluorescence signal at (a) 3531 A, (b) 3152 A, and (c) 3256 A. All spectra were produced with SOz concentration of = 2%. The dotted vertical runs through the shifted E band’s origin.
8, wavelengths corresponding to transitions from the upper level of both bands into the (400), ( 100) and (200) ground state levels, respectively. In curve (a) of Fig. 3, the excitation spectrum includes both neighboring bands, which implies that the nv; progression in the shifted E band fluorescence spectrum does exist and that its transition into the (400) level at 353 1 A, although coinciding with its intense counterpart of the E band system that appear in the background, has to some extent a measurable intensity. In curve (b ) of Fig. 3, the intensity of the shifted E band is negligible compared to that of the E band indicating that the peak at 3152 8, in curve (a) of Fig. 2 corresponds mainly to the transition into ( 100) from the E band as a background intensity, and that the Franck-Condon factor for the ( 100) member in the shifted E band system is very small. Finally, in the excitation spectrum of curve (c) of Fig. 3, the E band and the shifted E band appear with equal intensity at 2% concentration indicating that, in curve (a) of Fig. 2, about 50% of the intensity of the (200) member in the (nO0) progression is due to a background intensity from the transition into the ‘A1 (200) level in the E band system. The background intensity in the above example has a dramatic contribution because the (nO0) progression in curve (b) of Fig. 2 has a large intensity. For other transitions, the background intensity in curve (a) of Fig. 2 will be considerably lower but not equal throughout the spectrum. At higher SO1 concentra-
33-38
tion upper rotational levels of the E band will be also populated and the shifted E band’s fluorescence spectrum will be mutated even further. Therefore, the relative intensities of the transitions in curve (a) of Fig. 2 will not reflect the actual Franck-Condon patterns unless the concentration of SO* is low enough so that the two bands in Fig. 1 are well separated. For the E band’s fluorescence spectrum, on the other hand, the intermingling background intensity from the neighboring shifted E band will be, in general, negligible, especially for the strong transitions. However, there is another reason why its FranckCondon patterns will change as the SO;! concentration is varied and that is the presence of a hot band very near to its origin. Curve (c) of Fig. 2 shows the E band’s fluorescence spectrum excited by the same wavelength as in curve (b) but under lower SOz concentration ( < 1%). The relative intensities of the transitions in one spectrum now appear to be slightly different form the other, indicating that another band has also been excited by the same wavelength. According to ref. [ 12 1, this latter band has to be a hot band and, in fact, it can be assigned as the one sharing the same upper vibronic level with Clement’s L band, and having the lower level as ‘A, (010). This conclusion was supported further by noticing that the intensity ratio ZE/Z&i*d E was considerably larger at high concentration values (z 10%) than at low values ( 5 I%), which is a typical hot band behavior
[121At SOz concentration of zz 3% the E band’s fluorescence spectrum was found to be identical to the one produced with 2% concentration in terms of the transitions’ relative intensities. Therefore, although with the latter concentration close vibrational bands may not be very well resolved, the background intensity due to possible hot bands in the same region will be negligible. Also, the weak bands around 32855 cm-’ are reasonably far from the intense region of the E band under the 3% concentration condition and therefore their selectively excited fluorescence spectra at this concentration will reflect more or less their actual Franck-Condon patterns. The fluorescence spectra of three relatively weak bands around 32855 cm-’ are sown in Fig. 4 along with some identified lower ‘A, levels into which they decay. The vibrational analysis was carried out using the anharmonic parameters of ref. [ 141, which were
E. Hegazi et al. / Chemical PhysicsLetters221(1994) 33-38
3000
3200
3400
3600
3600
4000
4200
4400
4600
Angstrem
Fig. 4. Fluorescence spectra of excited weak bands at (a) 32850 cm-r, (b) 32853 cm-t, and (c) 32856 cm-‘. All spectra were produced with =3% concentration. The numbers identify the ‘A, (n, m, I) vibrational level into which transitions terminate.
32620
32630
32640
32850 32660 Wavenumber
32670 (cm-l)
32660
32690
32900
Fig. 5. Excitation spectra produced while monitoring the fluorescence signal at (a) 3721 A, (b) 3720 A, and (c) 3719 A. The SO1 concentration was 2% for all spectra. The fluorescence spectra in Fig. 4 were produced by exciting the marked bands in (b).
in fact useful in identifying the band origins to within f 1 cm-’ accuracy. For the fluorescence spectra of Fig. 4, those origins were determined as: (a) 32850 cm-‘, (b) 32853 cm-‘, and (c) 32856 cm-‘, and their corresponding bands can be seen separated from each other in curve (b) of Fig. 5, which depicts the excitation spectrum recorded while monitoring the fluorescence transition at 3720 A. The latter wavelength corresponds to a weak transition in the E band system and, at the same time, lies near strong transitions in the other bands’ systems. Similar excitation spectra are shown in curves (a) and (c) of Fig. 5 where the monochromator was set at 372 1 and 37 19
31
A, respectively. The change in the relative intensities in Fig. 5 is apparently occurring for cold vibrational bands in this region and not for the E band’s rotational structure. Excitation spectra similar to the ones shown in Fig. 5 could be produced such that every single vibrational band can be singled out from its neighbors, regardless of the resolution limit, by monitoring the proper fluorescence wavelength. We have compared several of these excitation spectra and have identified the following bands to be relatively the most intense ones: 32848, 32851, 32853, 32855, 32868, 32871,32877 and 32879 cm-’ ? 1 cm-’ in addition to the E band and the shifted E band. As pointed out in ref. [ lo], and seen here also, the investigated spectral range of the E band comprises of several adjacent vibrational levels that decay to the ‘AI (000) level. We find in the present work, however, that the upper levels of these bands do not all have the same vibronic symmetry. This can be seen by examining the vibrational transitions in the various resonance fluorescence spectra. For the E band fluorescence spectrum, transitions into levels having odd quanta of the unsymmetric mode v; are weak and limited only to the lowest levels such as 00 1, 10 1, 111, etc., and can be attributed to weak Coriolis coupling. The same features are also noticed for other bands such as the shifted E band and the 32856 cm-’ band in Fig. 4. Therefore, to fulfill the vibrational selection rules for the CzVpoint group, these bands’ upper levels must have either Br.a, or A2.b2 symmetry. The fluorescence spectra of the bands around 328 5 5 cm-‘, however, show relatively intense transitions into lower levels having odd quanta of v;. A typical example of such spectra is that of the excited 32856 cm-’ band, and to some extent the 32853 cm-’ band, which are shown in curves (a) and (b) of Fig. 4, respectively. These spectra show also transitions into lower levels of pure symmetric character, indicating that the vibronic symmetry of these bands’ upper levels, and possibly those of a few other bands in this region, is an admixture of Br.ar (or A,.b*) plus BI.bz (or A,.a, ) symmetries.
Acknowledgement Support from the research institute greatly appreciated.
of KFUPM
is
38
E. Hegazi et al. /Chemical PhysicsLetters221(1994) 33-38
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