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Optical-optical double resonance spectroscopy of the 1g(3P2)-A 3Π(1u)-X 1Σg+ transition of Cl2
Volume 159, number S,6
CHEMICAL PHYSICS LETTERS
OPTICAL-OPTICAL DOUBLE RESONANCE OF THE 1,(3P2)-A 31-I(l,,)-X ‘Z; TRANSITION Takashi ISHIWATA,
Atsushi ISHIGURO,
21 July 1989
SPECTROSCOPY OF Cl,
Kinichi OBI and Ikuzo TANAKA
Department of Chemistry, To&o Institute of Technology, Ohokayama. Meguro, Tobo 152, Japan
Received 17 February 1989; in fmal form 24 April 1989
We describe the results of optical-optical double-resonance spectroscopy of Cl2 on the 1,(3Pt)+- (hvl)-A%( 1,) + (hu, )-X ‘Zi transition. The state-selective transition from the ground state of Cl2 to the A ‘lI( I,) state was observed by scanning the pump laser ( hu, ) frequency, while furing the probe laser frequency ( hv2) to the ID( “P2)-A alI ( 1.) system. In this procedure, a rotational analysis on the A ‘IT( 1U1-X ‘I;: system is made on several vibrational bands for two isotopic species, ‘5C135CI and 37C135C1, and the vibrational numbering is determined from the isotope difference. The l,( ‘P2)-A ‘II( 1.) system is also analyzed fo establish the absolute position of the l,(“Pz) ion-pair state.
1. Introduction Until recently, the interpretation of the 258 nm emission band observed in a discharge remains a controversial subject in Cl2 spectroscopy [ I]. This system has been historically known as “E-B”, a transition from the ion-pair state designated as “E” to the well-known B llI (0: ) state [ 2,3 1. In our laboratory an attempt was made a few years ago to analyze the system by optical-optical double resonance through the B ‘II state as an intermediate [4]. As expected from the AJ2=0 selection rule of valence-ion-pair state transitions, the double-resonance spectra showed a transition terminating on the O,+(3PZ) state correlating to the lowest ionic states of C1-(‘S)+C1+(3PZ). The 0:(3P,)-B311(O~) band was then simulated to compare it with the 258 nm emission system. The profile of the fluorescence spectrum in the discharge resembled our simulation but its peak position was shifted to shorter wavelengths by about 1 nm. Our results were not consistent with a transient absorption spectrum taken by Briggs and Norrish [ 21, indicating a band system other than 0: ( 3Pz)-B ‘lT( 0: ) in the 258 nm region. Recently an important paper appeared in this respect. Tellinghuisen and co-workers re-examined the emission spectrum using Tesla discharge sources and observed many common bands with the transient 594
absorption spectrum [ 5 1. From the vibrational analysis, the lower state of this transition was concluded not to be the B ‘II(OZ ) state. In the analogy with other halogen molecules, the dominant emission near 258 nm was interpreted as the 2,(‘P2)-A’ 311(2,) transition, the rotational analysis of which was recently published [ 6 1. Among a number of bands which could not be assigned as the 2#P2)A’ ‘lI( 2,) system, they picked up the low Y’ progressions thought to be involved in another weak ionpair to valence transition, which was tentatively assigned as 1,(3P2)-A3n( 1,). However, the absolute positions for these four excited states were not clear, since none of the states involved in these two emission systems was observed in other systems. The purpose of this paper is to report a preliminary result of double-resonance experiments on the 1,(3PI)-A ‘l-I( 1,)-X ‘ZB’ photo-excitation sequence of Cl?. We rotationally analyze the two transitioncomponents, 1,(3P2)-A311(1,) andA311( 1,) -X ‘Cc, and determine the absolute position of the A’lT(l,) and l,(‘PZ) states. The analysis on the 18( 3PZ) -A 311( 1y ) system is completely consistent with the vibrational analysis of the weak emission at around 258 nm by Tellinghuisen and Chakraborty [ 5 1, and supports their assignment.
0 009-2614/89/$ ( North-Holland
03.50 Q Elsevier Science Publishers Physics Publishing Division )
B.V.
Volume 159, number 5,6
CHEMICAL PHYSICS LETTERS
2. Experimental
21 July 1989
rate of 10 Hz. Two dye lasers produced pulses with energy of around 5 mJ/puIse and 15 ns duration in the visible region, and the conversion efficiency to the ultraviolet was as low as 1%. There was no time delay between two laser pulses and the sample pressure of 3 Torr was low enough to exclude collisional relaxation in the intermediate A 311( 1.) state. The spectral bandwidth of the dye laser was 0.20.3 cm- ’ in broad band operation, while it could be reduced to 0.04 cm-’ by installing an etalon in the laser cavity. The laser frequency was calibrated by reference to the Iz B ‘II(O,’ )-X ‘Zz absorption spectrum with an estimated accuracy of 0.02 cm-‘.
The experimental apparatus used in this work was the same as that described previously [ 71. The double-resonance experiments were carried out by two tunable dye lasers pumped by the divided output of a XeCl excimer laser. The Clz molecules in the X ’ Ep’ ground state were first excited to the A ‘II ( 1U) state by a pump laser operated in the visible region (535-595 nm) and subsequently into the l,(‘Px) state by a probe laser. The probe laser was operated in the ultraviolet (248-259 nm) by mounting a KPB crystal on a crystal tilter, and the undesired fundamental output was removed by passing through a W separator. The double-resonance transitions were detected by monitoring the ultraviolet emission through a monochromator. Photomultiplier signals were amplified ten times by a preamplifier and averaged by a boxcar. The experiments were carried out at a repetition
3. Results and discussion 3.1. A 317(l,)-X ‘.Z$ Fig. 1a shows a typical example of the double-res-
a)
p9 Q
8 10 (
R
9 11 I
7
6 6I
I IQ I
5432 I
6
I
9 I
5
I
9 1
4
1
I I
3
,
I
6
5
'
"'1
21 17 4
32
1
I
19215
-A-
I
18220
19225
L-
hi’
Fig. I. (a) Doubl*reaonance spectrum showing the (7- 1) band of the A ‘II( 1.)-X ‘Z; system for 35CIJ5CI.The pump laser frequency (kv, ) was scanned by using an etalon tuning element with fixing the pmbe laser frequency (kv,=38638.3 cm-‘) close to the R head of the (O-7 ) band of the 1I( ‘P2)-A ‘II ( 1, ) system. (b ) The I2 B Tl (0.' )-X ‘q spectrum for the frequency calibration of the pump laser frequency.
595
Volume 159, number 5,6
CHEMICAL
PHYSICS LETTERS
onance spectra showing the A 311( 1, )-X ‘Zp’ system recorded through the ( 1 + 1) photo-excitation sequence:
(1) By reference to Tellinghuisen and Chakraborty’s results on the 1,(3P,)-A311( 1,) emission [5], we adjusted the probe laser frequency (v2) close to the R headofthe (O-7) transitionofthe 1,(3P2)-A311(l,) system for 35C135C1.Spectrum (a) was then recorded by scanning the pump laser with the intra-cavity etalon installed while monitoring the ultraviolet emission at 258 nm with a spectral slit width of 3 nm, where the 1J3P2)-A 311( 1,) emission was expected. It should be noted that the A 311( 1,)-X ‘Zz absorption system shows P, Q, and R triplets according to the AJ=O, k 1 selection rule as shown in fig. 1. This system is distinguishable from the B ‘II( 0: ) -X ‘Zg’ system consisting of the P and R doublet. The 1,(3Pz) and A ‘II( 1,) states should show an 52type splitting. However, the splitting was quite small at the lower Jlevels, and two types of transitions from the levels of the opposite parity obviously lie within
Table 1 A%( 1.)-X
‘2:
21 July 1989
the probe laser bandwidth under study ( = 0.2 cm - ’ ) . The double-resonance transition should share the common intermediate state in the P and R branches of the A 311( 1U)-X ‘Z: system corresponding to the following relation (neglecting the D, term): A,F”(J)=R(J’-I)-P(S-tl) =4B’:(J’+
l/2).
(2)
Since the molecular constants of the ground state were known accurately, these transitions were easily identified, including the vibrational quantum number of the ground state. Combined with the vibrational numbering of the A 311( 1,) state given by Tellinghuisen and Chakraborty [ 51, spectrum (a) in fig. 1 was assigned to the (7-l) band of the A311(l,)-X’C: system for the 35C135Clisotope species. Table 1 summarizes the results of the rotational analysis of the A311( 1,)-X ‘E,’ system for 35C135C1including 7 other vibrational bands (3 d v< 8 ) . The ( 3-2) band of the A 311( 1,)-X ‘Zz system was observed by exciting the molecule to the I,( ‘Pz) state near the R
system: 3sC1-3”C1
x
A-(1,)
‘z+I
0
v=o 3
v=l
u=2
VW@’
16895.94 0.1534 1.20x10-4
B b, F&’ 4
yoo & FC
17651.36 0.1503 3.49x 10-J
5
ho B” FC
17858.80 0.1471 8.02x 10-I
6
VW B” FC
7
VW B” FC
18226.79 0.1398 2.76~ 1O-4
8
lJo0 B” FC
18392.52 0.1357 4.34x 10-d
‘) Band origin (cm-‘).
596
b’ Rotational
18603.06 0.1437 9.95x LO-6
constant
(cm-‘).
c, Franck-Condon
18048.74 0.1436 1.%x 10-4
factor.
17499.83 0.1437 1.16x lo-’
Table 2 Preliminary spectroscopic constants for the A ?I( 1.) and l,(‘Pr) states of ‘sCl-3sCl ‘)
Table 3 A ‘IQ 1.)-X A’Wl,)
Yw Y10 Y*0 YW YO1 Y,, Y21
A3Nly)
l”(‘P2)
17453.25 (4) ‘) 252.772(23) -4.6172(44) -6.854(27)x 1O-2 0.16223(17) -2.087(67)x IO-’ -1.206(59)x10-’
57572.25(2) 252.206( 12) -1.0005(18)
0.116656(77) -6.29(27)x10-4
head of the (2-3) band of the l,(3Pz)-A311(1,) system at 40387.9 cm-‘, and the (4-l ) band of the A-X system was detected through the ( l-4) band of the 1,-A system at 39457.9 cm-‘, the (5-l) band through the (O-5) band at 39006.3 cm-‘, the (6-l ) band through the (O-6) band at 388 16.4 cm-‘, and the (8-l) band through the ( l-8) band at 38722.9 cm-‘. Table 2 shows the molecular constants de scribing the energies of the m-vibrational levels of 35C135C1observed in the experiment according to the usual expression: Y,k(~+l/2)‘[J(J+1)-l]k,
(3)
where the fitting was carried out by fixing the molecular constants of the ground state to the literature values [S]. We also extended the analysis to the 37C13-?Z1isotope species, the results of which are shown in table 3. These data cover the energy level range of 4 < V< 8 and 1 Q Jd 15. In these analyses, we confirmed the vibrational scheme of the A )II ( 1.) state suggested by Tellinghuisen and Chakraborty [ 5 ] from the isotope shifts of the band origins (Au& as shown in table 3: Au, = V& 3sC135C1)- V& 37C135C1) =C
Y~,k=O(l-p’)(~+1/2)‘+~Gy”,l.
‘2: system: “Cl-3sCl X’Z+I
V
L)Fittedtoeq. (3), 1r;J~15,and3~vd8fortheAJn(l,)state andOgudSforthe 1,(3P2)state. b, In cm-’ and 3u in parentheses.
T,,,=c
21 July 1989
CHEMICAL PHYSICS LETTERS
Volume 159, number 5,6
(4)
In this expression, p= (p37-35/$5-35 ) ‘I’= 0.98640 and the vibrational isotope shift in the 0” = 1 level of the ground state (iiG,. _ , = 11.26 cm-‘) was calculated from the published data [ 9 I. Prior to this work, there was no report on the
) b,
vzl
Au (obs.
Av(calc.)
4
17655.90 O’ 0.L465
1.46
1.43 c’
5
17855.35 0.1434
3.45
3.42
6
18043.64 0.1401
5.10
5.06
7
18220.41 0.1365
6.38
6.34
8
18385.23 0.1326
7.29
7.26
*’ In cm-‘. ‘) dv=~~(~‘Cl-~~Cl) - v,(“CI-“‘Cl). ‘) Assuming the isotope shift at the u= 1 level of the ground state tobe 11.26cm-‘.
A311( 1,)-X ‘Cl system in the absorption spectrum of Cl,. However, some evidence of the A %( 1,)-X ‘Cl system was found in the near-infrared emission spectra of the Cl atom recombination reaction and the NC& decomposition flame [ lo]. The plausible explanation was made on two vibrational progressions originating from the v’= 1 and 2 levels of the A % ( 1U) state based on the known intervals for the X ‘Cl state. The molecular constants in table 2 predicted the vibrational progression from u’= 1 to 0” = 8- 11 in the same spectral region (Q-10 cm-’ lower than the previous observation [lo] ). However, there is ambiguity in the assignment of the bands from the u’=2 level. These emission bands were observed 9-26 cm- ’ higher than our prediction for v’=2 to ~“~8-12. The progression from v’=O is located in the same range. The renumberingofthe (2-lo), (2-ll),and (2-12) bands to (O-9), (0-lO),and (0-ll),givesabetterfitwhile the assignments of the (2-8) and (2-9) bands remain valid (or they might be blended with the bands from v’ = 0 ) . This renumbering is supported by the calculation of the band intensities for the A3TI( 1,)-X ‘;CL emission assuming a thermal vibrational distribution in the A 311( 1,) state. Our Franck-Condon factor calculation for the A ‘rJ( 1,) -X ‘X2 system predicted the two weak and closelying (2-9) and (O-8) bands with comparable in597
Volume 159, number 5,6
CHEMICAL PHYSICS LETTERS
tensities, and the (O-9)-( O-l 1) bands with much higher intensities than the (2- 10 )- (2- 12 ) bands. The Franck-Condon factors of the A 311( l,)X ‘22 system shown in table 1 are quite small due to the large difference in their equilibrium internuclear distances (I,= 1.99 A for the X ‘Z$ state and 2.43 A for the A311(l,) state). Even though the B ‘II state lies 366 cm-* higher than the A 311( 1,) state, the dipole strength of the A %( 1,) -X ‘Cl system is expected to be an order of magnitude smaller than the B 311(0,’ )-X ‘C: system by analogy to other halogen molecules. It is obvious that the state selection by optical-optical double-resofacilitates the observation of nance the A 311(l,)-X ‘C: transition, which is weak and hidden by overlapping with many hot bands of the B ‘II ( 0: ) -X ‘Zp’ system. 3.2. Id3P2)-A
jli-(I,)
We also studied the l,( 3P2)-A 311( 1,) system by scanning the probe laser while the pump laser was tuned to an appropriate transition of the A ‘II( 1,)-X ‘El system. The double-resonance transitions were easily assigned according to the selection rule AJ= 0, f 1, while the Q branch was weak and missing in this parallel-type transition unless J was quite small. The emission spectra were also resolved to establish the absolute vibrational numbering of the 1g( 3P,) state consistent with Franck-Condon factor considerations for the 1,(‘P2)-A ‘II( 1,) system. The equilibrium internuclear distance of the ion-pair state of Clz ( r, = 2.9 A) is much larger than that of the A ‘II( 1,) state (r,=2.43 A). The low-u’ levels of the 1,(3Pz) state show the emission terminating on the high-v” levels of the A 3TI( 1U) state. The emission spectra are then modulated according to the probability distribution of the vibrational wavefunction of the 1,(3Pz) state: a u’= 0 level shows a single intensity maximum in the envelope of the spectrum; a v’= 1 shows two maxima; and so on. The U’= 0 level was identified as the first member of the vibrational progression in the double-resonance spectrum through the (7-l ) band of the A ‘l-I ( 1,)-X ‘El system. The envelope of the emission spectrum showed a single intensity maximum at ~“~6 of the A311( 1,) state. In the energy level analysis, the term values of the 1,(3P2) state were submitted to a global least-squares 598
21 July 1989
fitting in eq. ( 3) and the results are shown in table 2. The Q-type splitting of the l,(3P2) state and the centrifugal distortion term were not involved, since the observations were limited to JG 15. A rotational analysis was extended up to Y= 5 ( = 59000 cm-’ above the ground state), while we could not observe any double-resonance transitions other than the l,(3Pz)-A311( 1,) systemthroughtheA311( 1,) state in this energy range. Tellinghuisen and Chakrabony have recently reexamined the emission spectrum of Cl* using a Tesla discharge source containing isotopically pure 3sC135Cl and “C13’C1 [ 5 1. Apart from the dominant emission near 25 8 nm interpreted as 2, ( ‘Pz )-A’ ‘II (2, ) , they found a few other red-degraded bands, which were tentatively assigned as l,(3P2)-A ‘Tl( 1,). Of course it was the basis for our starting this work. The bandhead positions observed by Tellinghuisen and Chalcraborty agree with our analysis within 1.2 cm- t, and the absolute vibrational numbering of the l,(‘Pz) state is completely consistent. Finally, it should be pointed out that the identification of the A ‘II{ 1,)-X ‘Z: system gives a new impetus to Cl2 spectroscopy for analyzing the ion-pair states through the A )II( 1,) state. Such works are now in progress in this laboratory to elucidate the electronic structure of C12.
References [ 1] K.P. Huber and G. Henberg, Molecular spectra and molecular structure, Vol. 4. Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1978) pp. 146-149. [2] A.G. Brigs and R.G.W. No&h, Pmt. Roy. Sot. A 276 (1963) 51. [ 31 K. Wieland, J. Tellinghttisen and A, Nobs, J. Mol. Spectry. 41 (1972) 69. [4] T. Shinzawa, A. Tokunsga, T. Ishiwata and I. Tanaka, J. Chem. Phys. 83 (1985) 5407.
[ 51 J. Tellingbuisen and D.K. Chakmborty, Chem. Phys. Letters 134 (1987) 565. [6] P.C. Tellinghuisen, B. Guo, D.K. Chabraborty and J. Tellinghuisen, J. Mol. Spectty. 128 (1988) 268. [ 71 T. Ishiwata, T. Shinzawa, T. Kusayanagi and I. Tanaka, J. Chem. Phys. 82 (1985) 1788. [ 81J.A. Coxon, J. Mol. Spectry. 82 (1980) 264. [9]A.E.DouglasandA.R.Hoy,Can.J.Phys.53(1975) 1965. [ 101 J.A. Coxon, Low-lyidg excited states of diatomic halogen molecules, in: Molecular spectroscopy, Vol. 1 (Chem. Sot., London, 1973).
CHEMICAL PHYSICS LETTERS
OPTICAL-OPTICAL DOUBLE RESONANCE OF THE 1,(3P2)-A 31-I(l,,)-X ‘Z; TRANSITION Takashi ISHIWATA,
Atsushi ISHIGURO,
21 July 1989
SPECTROSCOPY OF Cl,
Kinichi OBI and Ikuzo TANAKA
Department of Chemistry, To&o Institute of Technology, Ohokayama. Meguro, Tobo 152, Japan
Received 17 February 1989; in fmal form 24 April 1989
We describe the results of optical-optical double-resonance spectroscopy of Cl2 on the 1,(3Pt)+- (hvl)-A%( 1,) + (hu, )-X ‘Zi transition. The state-selective transition from the ground state of Cl2 to the A ‘lI( I,) state was observed by scanning the pump laser ( hu, ) frequency, while furing the probe laser frequency ( hv2) to the ID( “P2)-A alI ( 1.) system. In this procedure, a rotational analysis on the A ‘IT( 1U1-X ‘I;: system is made on several vibrational bands for two isotopic species, ‘5C135CI and 37C135C1, and the vibrational numbering is determined from the isotope difference. The l,( ‘P2)-A ‘II( 1.) system is also analyzed fo establish the absolute position of the l,(“Pz) ion-pair state.
1. Introduction Until recently, the interpretation of the 258 nm emission band observed in a discharge remains a controversial subject in Cl2 spectroscopy [ I]. This system has been historically known as “E-B”, a transition from the ion-pair state designated as “E” to the well-known B llI (0: ) state [ 2,3 1. In our laboratory an attempt was made a few years ago to analyze the system by optical-optical double resonance through the B ‘II state as an intermediate [4]. As expected from the AJ2=0 selection rule of valence-ion-pair state transitions, the double-resonance spectra showed a transition terminating on the O,+(3PZ) state correlating to the lowest ionic states of C1-(‘S)+C1+(3PZ). The 0:(3P,)-B311(O~) band was then simulated to compare it with the 258 nm emission system. The profile of the fluorescence spectrum in the discharge resembled our simulation but its peak position was shifted to shorter wavelengths by about 1 nm. Our results were not consistent with a transient absorption spectrum taken by Briggs and Norrish [ 21, indicating a band system other than 0: ( 3Pz)-B ‘lT( 0: ) in the 258 nm region. Recently an important paper appeared in this respect. Tellinghuisen and co-workers re-examined the emission spectrum using Tesla discharge sources and observed many common bands with the transient 594
absorption spectrum [ 5 1. From the vibrational analysis, the lower state of this transition was concluded not to be the B ‘II(OZ ) state. In the analogy with other halogen molecules, the dominant emission near 258 nm was interpreted as the 2,(‘P2)-A’ 311(2,) transition, the rotational analysis of which was recently published [ 6 1. Among a number of bands which could not be assigned as the 2#P2)A’ ‘lI( 2,) system, they picked up the low Y’ progressions thought to be involved in another weak ionpair to valence transition, which was tentatively assigned as 1,(3P2)-A3n( 1,). However, the absolute positions for these four excited states were not clear, since none of the states involved in these two emission systems was observed in other systems. The purpose of this paper is to report a preliminary result of double-resonance experiments on the 1,(3PI)-A ‘l-I( 1,)-X ‘ZB’ photo-excitation sequence of Cl?. We rotationally analyze the two transitioncomponents, 1,(3P2)-A311(1,) andA311( 1,) -X ‘Cc, and determine the absolute position of the A’lT(l,) and l,(‘PZ) states. The analysis on the 18( 3PZ) -A 311( 1y ) system is completely consistent with the vibrational analysis of the weak emission at around 258 nm by Tellinghuisen and Chakraborty [ 5 1, and supports their assignment.
0 009-2614/89/$ ( North-Holland
03.50 Q Elsevier Science Publishers Physics Publishing Division )
B.V.
Volume 159, number 5,6
CHEMICAL PHYSICS LETTERS
2. Experimental
21 July 1989
rate of 10 Hz. Two dye lasers produced pulses with energy of around 5 mJ/puIse and 15 ns duration in the visible region, and the conversion efficiency to the ultraviolet was as low as 1%. There was no time delay between two laser pulses and the sample pressure of 3 Torr was low enough to exclude collisional relaxation in the intermediate A 311( 1.) state. The spectral bandwidth of the dye laser was 0.20.3 cm- ’ in broad band operation, while it could be reduced to 0.04 cm-’ by installing an etalon in the laser cavity. The laser frequency was calibrated by reference to the Iz B ‘II(O,’ )-X ‘Zz absorption spectrum with an estimated accuracy of 0.02 cm-‘.
The experimental apparatus used in this work was the same as that described previously [ 71. The double-resonance experiments were carried out by two tunable dye lasers pumped by the divided output of a XeCl excimer laser. The Clz molecules in the X ’ Ep’ ground state were first excited to the A ‘II ( 1U) state by a pump laser operated in the visible region (535-595 nm) and subsequently into the l,(‘Px) state by a probe laser. The probe laser was operated in the ultraviolet (248-259 nm) by mounting a KPB crystal on a crystal tilter, and the undesired fundamental output was removed by passing through a W separator. The double-resonance transitions were detected by monitoring the ultraviolet emission through a monochromator. Photomultiplier signals were amplified ten times by a preamplifier and averaged by a boxcar. The experiments were carried out at a repetition
3. Results and discussion 3.1. A 317(l,)-X ‘.Z$ Fig. 1a shows a typical example of the double-res-
a)
p9 Q
8 10 (
R
9 11 I
7
6 6I
I IQ I
5432 I
6
I
9 I
5
I
9 1
4
1
I I
3
,
I
6
5
'
"'1
21 17 4
32
1
I
19215
-A-
I
18220
19225
L-
hi’
Fig. I. (a) Doubl*reaonance spectrum showing the (7- 1) band of the A ‘II( 1.)-X ‘Z; system for 35CIJ5CI.The pump laser frequency (kv, ) was scanned by using an etalon tuning element with fixing the pmbe laser frequency (kv,=38638.3 cm-‘) close to the R head of the (O-7 ) band of the 1I( ‘P2)-A ‘II ( 1, ) system. (b ) The I2 B Tl (0.' )-X ‘q spectrum for the frequency calibration of the pump laser frequency.
595
Volume 159, number 5,6
CHEMICAL
PHYSICS LETTERS
onance spectra showing the A 311( 1, )-X ‘Zp’ system recorded through the ( 1 + 1) photo-excitation sequence:
(1) By reference to Tellinghuisen and Chakraborty’s results on the 1,(3P,)-A311( 1,) emission [5], we adjusted the probe laser frequency (v2) close to the R headofthe (O-7) transitionofthe 1,(3P2)-A311(l,) system for 35C135C1.Spectrum (a) was then recorded by scanning the pump laser with the intra-cavity etalon installed while monitoring the ultraviolet emission at 258 nm with a spectral slit width of 3 nm, where the 1J3P2)-A 311( 1,) emission was expected. It should be noted that the A 311( 1,)-X ‘Zz absorption system shows P, Q, and R triplets according to the AJ=O, k 1 selection rule as shown in fig. 1. This system is distinguishable from the B ‘II( 0: ) -X ‘Zg’ system consisting of the P and R doublet. The 1,(3Pz) and A ‘II( 1,) states should show an 52type splitting. However, the splitting was quite small at the lower Jlevels, and two types of transitions from the levels of the opposite parity obviously lie within
Table 1 A%( 1.)-X
‘2:
21 July 1989
the probe laser bandwidth under study ( = 0.2 cm - ’ ) . The double-resonance transition should share the common intermediate state in the P and R branches of the A 311( 1U)-X ‘Z: system corresponding to the following relation (neglecting the D, term): A,F”(J)=R(J’-I)-P(S-tl) =4B’:(J’+
l/2).
(2)
Since the molecular constants of the ground state were known accurately, these transitions were easily identified, including the vibrational quantum number of the ground state. Combined with the vibrational numbering of the A 311( 1,) state given by Tellinghuisen and Chakraborty [ 51, spectrum (a) in fig. 1 was assigned to the (7-l) band of the A311(l,)-X’C: system for the 35C135Clisotope species. Table 1 summarizes the results of the rotational analysis of the A311( 1,)-X ‘E,’ system for 35C135C1including 7 other vibrational bands (3 d v< 8 ) . The ( 3-2) band of the A 311( 1,)-X ‘Zz system was observed by exciting the molecule to the I,( ‘Pz) state near the R
system: 3sC1-3”C1
x
A-(1,)
‘z+I
0
v=o 3
v=l
u=2
VW@’
16895.94 0.1534 1.20x10-4
B b, F&’ 4
yoo & FC
17651.36 0.1503 3.49x 10-J
5
ho B” FC
17858.80 0.1471 8.02x 10-I
6
VW B” FC
7
VW B” FC
18226.79 0.1398 2.76~ 1O-4
8
lJo0 B” FC
18392.52 0.1357 4.34x 10-d
‘) Band origin (cm-‘).
596
b’ Rotational
18603.06 0.1437 9.95x LO-6
constant
(cm-‘).
c, Franck-Condon
18048.74 0.1436 1.%x 10-4
factor.
17499.83 0.1437 1.16x lo-’
Table 2 Preliminary spectroscopic constants for the A ?I( 1.) and l,(‘Pr) states of ‘sCl-3sCl ‘)
Table 3 A ‘IQ 1.)-X A’Wl,)
Yw Y10 Y*0 YW YO1 Y,, Y21
A3Nly)
l”(‘P2)
17453.25 (4) ‘) 252.772(23) -4.6172(44) -6.854(27)x 1O-2 0.16223(17) -2.087(67)x IO-’ -1.206(59)x10-’
57572.25(2) 252.206( 12) -1.0005(18)
0.116656(77) -6.29(27)x10-4
head of the (2-3) band of the l,(3Pz)-A311(1,) system at 40387.9 cm-‘, and the (4-l ) band of the A-X system was detected through the ( l-4) band of the 1,-A system at 39457.9 cm-‘, the (5-l) band through the (O-5) band at 39006.3 cm-‘, the (6-l ) band through the (O-6) band at 388 16.4 cm-‘, and the (8-l) band through the ( l-8) band at 38722.9 cm-‘. Table 2 shows the molecular constants de scribing the energies of the m-vibrational levels of 35C135C1observed in the experiment according to the usual expression: Y,k(~+l/2)‘[J(J+1)-l]k,
(3)
where the fitting was carried out by fixing the molecular constants of the ground state to the literature values [S]. We also extended the analysis to the 37C13-?Z1isotope species, the results of which are shown in table 3. These data cover the energy level range of 4 < V< 8 and 1 Q Jd 15. In these analyses, we confirmed the vibrational scheme of the A )II ( 1.) state suggested by Tellinghuisen and Chakraborty [ 5 ] from the isotope shifts of the band origins (Au& as shown in table 3: Au, = V& 3sC135C1)- V& 37C135C1) =C
Y~,k=O(l-p’)(~+1/2)‘+~Gy”,l.
‘2: system: “Cl-3sCl X’Z+I
V
L)Fittedtoeq. (3), 1r;J~15,and3~vd8fortheAJn(l,)state andOgudSforthe 1,(3P2)state. b, In cm-’ and 3u in parentheses.
T,,,=c
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CHEMICAL PHYSICS LETTERS
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(4)
In this expression, p= (p37-35/$5-35 ) ‘I’= 0.98640 and the vibrational isotope shift in the 0” = 1 level of the ground state (iiG,. _ , = 11.26 cm-‘) was calculated from the published data [ 9 I. Prior to this work, there was no report on the
) b,
vzl
Au (obs.
Av(calc.)
4
17655.90 O’ 0.L465
1.46
1.43 c’
5
17855.35 0.1434
3.45
3.42
6
18043.64 0.1401
5.10
5.06
7
18220.41 0.1365
6.38
6.34
8
18385.23 0.1326
7.29
7.26
*’ In cm-‘. ‘) dv=~~(~‘Cl-~~Cl) - v,(“CI-“‘Cl). ‘) Assuming the isotope shift at the u= 1 level of the ground state tobe 11.26cm-‘.
A311( 1,)-X ‘Cl system in the absorption spectrum of Cl,. However, some evidence of the A %( 1,)-X ‘Cl system was found in the near-infrared emission spectra of the Cl atom recombination reaction and the NC& decomposition flame [ lo]. The plausible explanation was made on two vibrational progressions originating from the v’= 1 and 2 levels of the A % ( 1U) state based on the known intervals for the X ‘Cl state. The molecular constants in table 2 predicted the vibrational progression from u’= 1 to 0” = 8- 11 in the same spectral region (Q-10 cm-’ lower than the previous observation [lo] ). However, there is ambiguity in the assignment of the bands from the u’=2 level. These emission bands were observed 9-26 cm- ’ higher than our prediction for v’=2 to ~“~8-12. The progression from v’=O is located in the same range. The renumberingofthe (2-lo), (2-ll),and (2-12) bands to (O-9), (0-lO),and (0-ll),givesabetterfitwhile the assignments of the (2-8) and (2-9) bands remain valid (or they might be blended with the bands from v’ = 0 ) . This renumbering is supported by the calculation of the band intensities for the A3TI( 1,)-X ‘;CL emission assuming a thermal vibrational distribution in the A 311( 1,) state. Our Franck-Condon factor calculation for the A ‘rJ( 1,) -X ‘X2 system predicted the two weak and closelying (2-9) and (O-8) bands with comparable in597
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CHEMICAL PHYSICS LETTERS
tensities, and the (O-9)-( O-l 1) bands with much higher intensities than the (2- 10 )- (2- 12 ) bands. The Franck-Condon factors of the A 311( l,)X ‘22 system shown in table 1 are quite small due to the large difference in their equilibrium internuclear distances (I,= 1.99 A for the X ‘Z$ state and 2.43 A for the A311(l,) state). Even though the B ‘II state lies 366 cm-* higher than the A 311( 1,) state, the dipole strength of the A %( 1,) -X ‘Cl system is expected to be an order of magnitude smaller than the B 311(0,’ )-X ‘C: system by analogy to other halogen molecules. It is obvious that the state selection by optical-optical double-resofacilitates the observation of nance the A 311(l,)-X ‘C: transition, which is weak and hidden by overlapping with many hot bands of the B ‘II ( 0: ) -X ‘Zp’ system. 3.2. Id3P2)-A
jli-(I,)
We also studied the l,( 3P2)-A 311( 1,) system by scanning the probe laser while the pump laser was tuned to an appropriate transition of the A ‘II( 1,)-X ‘El system. The double-resonance transitions were easily assigned according to the selection rule AJ= 0, f 1, while the Q branch was weak and missing in this parallel-type transition unless J was quite small. The emission spectra were also resolved to establish the absolute vibrational numbering of the 1g( 3P,) state consistent with Franck-Condon factor considerations for the 1,(‘P2)-A ‘II( 1,) system. The equilibrium internuclear distance of the ion-pair state of Clz ( r, = 2.9 A) is much larger than that of the A ‘II( 1,) state (r,=2.43 A). The low-u’ levels of the 1,(3Pz) state show the emission terminating on the high-v” levels of the A 3TI( 1U) state. The emission spectra are then modulated according to the probability distribution of the vibrational wavefunction of the 1,(3Pz) state: a u’= 0 level shows a single intensity maximum in the envelope of the spectrum; a v’= 1 shows two maxima; and so on. The U’= 0 level was identified as the first member of the vibrational progression in the double-resonance spectrum through the (7-l ) band of the A ‘l-I ( 1,)-X ‘El system. The envelope of the emission spectrum showed a single intensity maximum at ~“~6 of the A311( 1,) state. In the energy level analysis, the term values of the 1,(3P2) state were submitted to a global least-squares 598
21 July 1989
fitting in eq. ( 3) and the results are shown in table 2. The Q-type splitting of the l,(3P2) state and the centrifugal distortion term were not involved, since the observations were limited to JG 15. A rotational analysis was extended up to Y= 5 ( = 59000 cm-’ above the ground state), while we could not observe any double-resonance transitions other than the l,(3Pz)-A311( 1,) systemthroughtheA311( 1,) state in this energy range. Tellinghuisen and Chakrabony have recently reexamined the emission spectrum of Cl* using a Tesla discharge source containing isotopically pure 3sC135Cl and “C13’C1 [ 5 1. Apart from the dominant emission near 25 8 nm interpreted as 2, ( ‘Pz )-A’ ‘II (2, ) , they found a few other red-degraded bands, which were tentatively assigned as l,(3P2)-A ‘Tl( 1,). Of course it was the basis for our starting this work. The bandhead positions observed by Tellinghuisen and Chalcraborty agree with our analysis within 1.2 cm- t, and the absolute vibrational numbering of the l,(‘Pz) state is completely consistent. Finally, it should be pointed out that the identification of the A ‘II{ 1,)-X ‘Z: system gives a new impetus to Cl2 spectroscopy for analyzing the ion-pair states through the A )II( 1,) state. Such works are now in progress in this laboratory to elucidate the electronic structure of C12.
References [ 1] K.P. Huber and G. Henberg, Molecular spectra and molecular structure, Vol. 4. Constants of diatomic molecules (Van Nostrand Reinhold, New York, 1978) pp. 146-149. [2] A.G. Brigs and R.G.W. No&h, Pmt. Roy. Sot. A 276 (1963) 51. [ 31 K. Wieland, J. Tellinghttisen and A, Nobs, J. Mol. Spectry. 41 (1972) 69. [4] T. Shinzawa, A. Tokunsga, T. Ishiwata and I. Tanaka, J. Chem. Phys. 83 (1985) 5407.
[ 51 J. Tellingbuisen and D.K. Chakmborty, Chem. Phys. Letters 134 (1987) 565. [6] P.C. Tellinghuisen, B. Guo, D.K. Chabraborty and J. Tellinghuisen, J. Mol. Spectty. 128 (1988) 268. [ 71 T. Ishiwata, T. Shinzawa, T. Kusayanagi and I. Tanaka, J. Chem. Phys. 82 (1985) 1788. [ 81J.A. Coxon, J. Mol. Spectry. 82 (1980) 264. [9]A.E.DouglasandA.R.Hoy,Can.J.Phys.53(1975) 1965. [ 101 J.A. Coxon, Low-lyidg excited states of diatomic halogen molecules, in: Molecular spectroscopy, Vol. 1 (Chem. Sot., London, 1973).