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Microwave transitions from the Ã1A2 excited state of thioformaldehyde to high rovibronic levels of the X̃ 1A1 ground state
Volume 118,number
1
CHEMICAL
PHYSICS
12 July 1985
LETTERS
MICROWAVE TRANSITIONS FROM THE A’A, EXCITED STATE OF THIOFOP~LDEHYDE TO HIGH ROVIBRONIC LEVELS OF THE x ‘A, GROUND STATE J.C. PETERSEN HeRberg
Immure
’ and D.A.
of Asrrophjsrcs,
RAMSAY Narionol Research Councrl o/ Canad
Orrawz, Onrmio.
Canada KiA
OR6
Received 5 April 1985
Microwave-opdcal double rcsonan~~ studies have been carried out on four rotational levels of Lhe A’A, sue of rhioformaldehyde (H,CS). Several double re~~nancc signals have been observed which involve microwave transirions between levels of the excited state and high rovibronic levels of rhc ground stale. Inreresling Zeeman erfecls are observed.
1. Introduction Recent studres of the x1A2-% lA1 system of thioformaldehyde by inter-modulated fluorescence and by microwave-optrcal double resonance (MODR) have revealed the presence of numerous small perturbations
r.n the excited-state
tions with high rovibronic [ 1,2]_ excited
levels caused by interaclevels of the ground
state
Similar perturbations are found in the x IA” states of HNO and DNO [3,4] _The perturba-
tions normahy range in magnitude from a few to several hundred MHz and limit the accuracy with which molecular constants can be determined in electronically excited states. In carrying out the PvlODR experiments we frequently observe many microwave lines when a single optical transition is pumped. Some of these microwave frequencies can be assigned to rotational transitions in the ground or excited states involving either the levels pumped by the laser or levels connected to these by colhsronal relaxation processes [5]. In the present paper we shall ahow that some of the unassigned microwave lines arise from transitions between rovibronic levels of the excited state and high rovibronic levels of the -ground state. Similar transitions have been reported in XODR studies on NH, [6,7] _
’
NRCC Research
Table 1 Microwave frequencies observed in double resonance expeximents involving roviixonic levels in the x ’ A2 excited state of thioformaldehyde optical tiansitions
Microwave frequencies observed (hLHz)
pump& 164-163 and 164-153
8150.6 8702.3 9717.3 (11727 3) a)
8550 3 8933.9 9741.3 11745.1
122,10-121 .ll and 122,10-Ill,11
8313.6 9493.6 10784.2 (11465.1) e)
(~10.2) b) 97455 (10832.1) =I
8990.2 10540.8 (10887.2)dj
=2,11-121,lZ
(8990.2)r) 99014 (10887.2) a)
9482.6 (10062.9) =) (11465.1)=)
9526.0 (10832.1)=)
100,10--Lo1,9
(8170.8) 9) 9376.7 (11147.0)~
8476.8 100669 11345.2
8595.4 9615 6 11030.0 11951.2
(8644.4)ti 107293
a) This line ti equally strong when 174 is pumped. b)c0lhsionalsignaZ~g~omi50,15d C0rhtional~arising&om11a 9. d)cotiotisignalarinnghom132:11. e) weak signal, 0ngi0 ~uoertain. r) couisiohrd signal arisiq from 12710 9) collisionalsignal~ fiom90,9_-
Asociale.
31
Volume
CHEMICAL
118. number 1
PHYSICS
2. ExperLnenti The experimental conditions for carrying out the rmcrowave-ptical double resonance experiments have been described earlier [2] _ In the present experiments four rotibronic levels in the 4l level of the x lA2 excited state were selected for study, viz. 164, 12,,,,, 122,11 and lOo,,,-,. The optical transitions pumped are given in table 1; the pumping powers ranged from 200 to 300 mW. The microwave source was swept from 8 to 12 GHz and MODR signals were recorded with a time constant of 0.1 s. Microwave powers up to 5 W were used. The microwave Frequencres are listed in table 1 and are accurate to a few tenths of a MHz. Some of the frequencies were found to originate from levels populated by collisional relaxation from the level pumped. These frequencies are included in parentheses and were Identified by fixing
b
LE-lTERS
12 July
1985
the microwave frequency and scanning the laser over lines involving neighbouring J and Ka levels [5] _ Further experiments were carried out in a magnetic field with a flux density of 275 G. The direction of the magnetic field was at nght angles to the electric vectors of the optical and microwave beams. All the lines were found to split into doublets with separations ranging from 3 to 27 MHz except for the line at 10540.8 MHz which broadens. Two examples are shown in fig. l_
3. JXscus5ion The rotatlonal constants for the ground and excited states of tbioformaldehyde are well known [S] and it is readily verified that the microwave frequencies in table 1 do not correspond to rotational transitions within these states, involving either the levels pumped
d
A
Fig. 1. Two MODR lines originating from the 164 rotatioul level of the x ‘A 1, v4 = 1 state of thioformaldehyder (a, c) no magnetic field, (b, d) magnetic field with fhuc density of 275 G in perpendicular polarisxtion The vertical scak in (b) and (d) me 2; times larger than in (a) and(c).
32
Volume 118. numba
1
CHEMICAL
PHYSICS
or levels which may be affected by collisional relaxation procc=zes. The frequencies must therefore correspond to transitions from the excited state levels to either neighbouring triplet levels or to high rovibronic levels of the ground state_ The majority of the lines originate in the level pumped by the laser, but some or&rate in other levels populated by collisional relaxation (table 1) All the MODR signals have the same sign which corresponds to a decrease in the total fluorescence when the microwave power is on. These results are consistent with the pumping of molecules by microwave radiation from a level which fluoresces to a level with a very nnall fluorescence efficiency. The magnetic effects are much smaller than those which have been observed in earlier work for rnicrowave transitions involving triplet levels [3] or singlet levels appreciably perturbed by tnplet levels [2] We therefore conclude that the microwave frequencies correspond to transitions between rovibronic levels of the x lA2 state and high rovibronic levels of the ground state. The splittings of the lines into two components m the presence of a magnetic field in perpendicular polansation are similar to those found in Zeeman studies of the ground state of formaldehyde [9,10] _However, the splittings observed in the present work are larger by at least an order of magnitude, indicating that the levels involved have much larger rotational magnetic moments or nnall admixtures of triplet character. The density of ground state levels in the region of the x 1 A2 excited state of thioformaldehyde is calculated to be 1.6 levels per cm-l on the basis of the known vibrational frequencies [ 1 l] and a harmonic approximation. Smce the excited state has Ba vibronic symmetry (=A2 X vbl), microwave transitions with a-, b- and c-type selection rules are potible to high levels of the ground state with B2, A, and A2 dblational symmeties respectively_ The rotational selection rules are N = 0, Al, AK, = 0 for the a-type transitions, and N = 0, *I, A& = 21 for b- and c-type transitions. Since the number of g-round-state vibra-
LEITERS
12 July 1985
tional 1eveIs in each of the four symmetry classes is approximately the same at 17000 cm-l, the number of microwave transitions in a frequency range of 1 cm-1 is expected to be of the order of 0.75 K 1.6 X 15 = 18. If higher-order transitions are observed, or if K, is not a good quantum number, a larger number may be expected. Experimentally we find an average number of 5 transitions in a frequency range of 4 GI-Iz, due allowance bemg made for the two components of the 164 level which are pumped by the laser. The agreement between the observed and calculated numbers can be considered to be satisfactory in view of the assumptions involved_ Microwave--optical double resonance therefore provides a method for probing high rovibronic levels in the ground states of molecules. Although it is possible that rotational quantum numbers might be established on the basis of further experiments, it is unlikely that much progress will be made in elucidating the vibrational levels involved
References III K-H. Fung and DA. Ramsay, J. Phys. Chem. 88 (1984) 395.
121 J.C. Petersen, D.A. Ramsay and T. Amano, Chem
Phys. Letters 103 (1984) 266. 131 J.C. Petersen, S. Stio, T. Amauo and DA_ Ramsay, Can. J. Phys. 62 (1984) 1731. 141 J.C. Petersen, T. Amano and DA. Ramsay, I. Chem.
Phys. 81 (1984) 5449_ 151 J.C. Petersen and DA. Ramsay, Chem. phys_ Letters 118 (1985) 34. 161 G-W. Hills and R-F. Curl Jr.. 1. Chem. Phys. 66 (1977) 1507. 171 G.W. Hills, J. MoL Specby. 93 (1982) 395. 181 DJ. Clouthier, D.C. Moule, DA. Ramsay and
F.W. Birss, &II. J. Phys. 60 (1982) 1212. H. Hir&awa, A. M~yahara, T. Oka and K. Shimoda, J. Phys. Sot Japan 15 (1960) 303.
I91 K. Kondo,
J. Chem. Phys. 42 (1965) 1563. r101 W. Fly-, 1111 DJ. Clouthier and DA. Ramsay, Ann Rev. Phys. (Ihem. 34 (1983) 31.
33
1
CHEMICAL
PHYSICS
12 July 1985
LETTERS
MICROWAVE TRANSITIONS FROM THE A’A, EXCITED STATE OF THIOFOP~LDEHYDE TO HIGH ROVIBRONIC LEVELS OF THE x ‘A, GROUND STATE J.C. PETERSEN HeRberg
Immure
’ and D.A.
of Asrrophjsrcs,
RAMSAY Narionol Research Councrl o/ Canad
Orrawz, Onrmio.
Canada KiA
OR6
Received 5 April 1985
Microwave-opdcal double rcsonan~~ studies have been carried out on four rotational levels of Lhe A’A, sue of rhioformaldehyde (H,CS). Several double re~~nancc signals have been observed which involve microwave transirions between levels of the excited state and high rovibronic levels of rhc ground stale. Inreresling Zeeman erfecls are observed.
1. Introduction Recent studres of the x1A2-% lA1 system of thioformaldehyde by inter-modulated fluorescence and by microwave-optrcal double resonance (MODR) have revealed the presence of numerous small perturbations
r.n the excited-state
tions with high rovibronic [ 1,2]_ excited
levels caused by interaclevels of the ground
state
Similar perturbations are found in the x IA” states of HNO and DNO [3,4] _The perturba-
tions normahy range in magnitude from a few to several hundred MHz and limit the accuracy with which molecular constants can be determined in electronically excited states. In carrying out the PvlODR experiments we frequently observe many microwave lines when a single optical transition is pumped. Some of these microwave frequencies can be assigned to rotational transitions in the ground or excited states involving either the levels pumped by the laser or levels connected to these by colhsronal relaxation processes [5]. In the present paper we shall ahow that some of the unassigned microwave lines arise from transitions between rovibronic levels of the excited state and high rovibronic levels of the -ground state. Similar transitions have been reported in XODR studies on NH, [6,7] _
’
NRCC Research
Table 1 Microwave frequencies observed in double resonance expeximents involving roviixonic levels in the x ’ A2 excited state of thioformaldehyde optical tiansitions
Microwave frequencies observed (hLHz)
pump& 164-163 and 164-153
8150.6 8702.3 9717.3 (11727 3) a)
8550 3 8933.9 9741.3 11745.1
122,10-121 .ll and 122,10-Ill,11
8313.6 9493.6 10784.2 (11465.1) e)
(~10.2) b) 97455 (10832.1) =I
8990.2 10540.8 (10887.2)dj
=2,11-121,lZ
(8990.2)r) 99014 (10887.2) a)
9482.6 (10062.9) =) (11465.1)=)
9526.0 (10832.1)=)
100,10--Lo1,9
(8170.8) 9) 9376.7 (11147.0)~
8476.8 100669 11345.2
8595.4 9615 6 11030.0 11951.2
(8644.4)ti 107293
a) This line ti equally strong when 174 is pumped. b)c0lhsionalsignaZ~g~omi50,15d C0rhtional~arising&om11a 9. d)cotiotisignalarinnghom132:11. e) weak signal, 0ngi0 ~uoertain. r) couisiohrd signal arisiq from 12710 9) collisionalsignal~ fiom90,9_-
Asociale.
31
Volume
CHEMICAL
118. number 1
PHYSICS
2. ExperLnenti The experimental conditions for carrying out the rmcrowave-ptical double resonance experiments have been described earlier [2] _ In the present experiments four rotibronic levels in the 4l level of the x lA2 excited state were selected for study, viz. 164, 12,,,,, 122,11 and lOo,,,-,. The optical transitions pumped are given in table 1; the pumping powers ranged from 200 to 300 mW. The microwave source was swept from 8 to 12 GHz and MODR signals were recorded with a time constant of 0.1 s. Microwave powers up to 5 W were used. The microwave Frequencres are listed in table 1 and are accurate to a few tenths of a MHz. Some of the frequencies were found to originate from levels populated by collisional relaxation from the level pumped. These frequencies are included in parentheses and were Identified by fixing
b
LE-lTERS
12 July
1985
the microwave frequency and scanning the laser over lines involving neighbouring J and Ka levels [5] _ Further experiments were carried out in a magnetic field with a flux density of 275 G. The direction of the magnetic field was at nght angles to the electric vectors of the optical and microwave beams. All the lines were found to split into doublets with separations ranging from 3 to 27 MHz except for the line at 10540.8 MHz which broadens. Two examples are shown in fig. l_
3. JXscus5ion The rotatlonal constants for the ground and excited states of tbioformaldehyde are well known [S] and it is readily verified that the microwave frequencies in table 1 do not correspond to rotational transitions within these states, involving either the levels pumped
d
A
Fig. 1. Two MODR lines originating from the 164 rotatioul level of the x ‘A 1, v4 = 1 state of thioformaldehyder (a, c) no magnetic field, (b, d) magnetic field with fhuc density of 275 G in perpendicular polarisxtion The vertical scak in (b) and (d) me 2; times larger than in (a) and(c).
32
Volume 118. numba
1
CHEMICAL
PHYSICS
or levels which may be affected by collisional relaxation procc=zes. The frequencies must therefore correspond to transitions from the excited state levels to either neighbouring triplet levels or to high rovibronic levels of the ground state_ The majority of the lines originate in the level pumped by the laser, but some or&rate in other levels populated by collisional relaxation (table 1) All the MODR signals have the same sign which corresponds to a decrease in the total fluorescence when the microwave power is on. These results are consistent with the pumping of molecules by microwave radiation from a level which fluoresces to a level with a very nnall fluorescence efficiency. The magnetic effects are much smaller than those which have been observed in earlier work for rnicrowave transitions involving triplet levels [3] or singlet levels appreciably perturbed by tnplet levels [2] We therefore conclude that the microwave frequencies correspond to transitions between rovibronic levels of the x lA2 state and high rovibronic levels of the ground state. The splittings of the lines into two components m the presence of a magnetic field in perpendicular polansation are similar to those found in Zeeman studies of the ground state of formaldehyde [9,10] _However, the splittings observed in the present work are larger by at least an order of magnitude, indicating that the levels involved have much larger rotational magnetic moments or nnall admixtures of triplet character. The density of ground state levels in the region of the x 1 A2 excited state of thioformaldehyde is calculated to be 1.6 levels per cm-l on the basis of the known vibrational frequencies [ 1 l] and a harmonic approximation. Smce the excited state has Ba vibronic symmetry (=A2 X vbl), microwave transitions with a-, b- and c-type selection rules are potible to high levels of the ground state with B2, A, and A2 dblational symmeties respectively_ The rotational selection rules are N = 0, Al, AK, = 0 for the a-type transitions, and N = 0, *I, A& = 21 for b- and c-type transitions. Since the number of g-round-state vibra-
LEITERS
12 July 1985
tional 1eveIs in each of the four symmetry classes is approximately the same at 17000 cm-l, the number of microwave transitions in a frequency range of 1 cm-1 is expected to be of the order of 0.75 K 1.6 X 15 = 18. If higher-order transitions are observed, or if K, is not a good quantum number, a larger number may be expected. Experimentally we find an average number of 5 transitions in a frequency range of 4 GI-Iz, due allowance bemg made for the two components of the 164 level which are pumped by the laser. The agreement between the observed and calculated numbers can be considered to be satisfactory in view of the assumptions involved_ Microwave--optical double resonance therefore provides a method for probing high rovibronic levels in the ground states of molecules. Although it is possible that rotational quantum numbers might be established on the basis of further experiments, it is unlikely that much progress will be made in elucidating the vibrational levels involved
References III K-H. Fung and DA. Ramsay, J. Phys. Chem. 88 (1984) 395.
121 J.C. Petersen, D.A. Ramsay and T. Amano, Chem
Phys. Letters 103 (1984) 266. 131 J.C. Petersen, S. Stio, T. Amauo and DA_ Ramsay, Can. J. Phys. 62 (1984) 1731. 141 J.C. Petersen, T. Amano and DA. Ramsay, I. Chem.
Phys. 81 (1984) 5449_ 151 J.C. Petersen and DA. Ramsay, Chem. phys_ Letters 118 (1985) 34. 161 G-W. Hills and R-F. Curl Jr.. 1. Chem. Phys. 66 (1977) 1507. 171 G.W. Hills, J. MoL Specby. 93 (1982) 395. 181 DJ. Clouthier, D.C. Moule, DA. Ramsay and
F.W. Birss, &II. J. Phys. 60 (1982) 1212. H. Hir&awa, A. M~yahara, T. Oka and K. Shimoda, J. Phys. Sot Japan 15 (1960) 303.
I91 K. Kondo,
J. Chem. Phys. 42 (1965) 1563. r101 W. Fly-, 1111 DJ. Clouthier and DA. Ramsay, Ann Rev. Phys. (Ihem. 34 (1983) 31.
33