- Email: [email protected]
Laser-induced fluorescence spectra of free-jet cooled organic free radicals. Vinoxy, cyclopentadienyl, and benzyl
\‘olurne95. number4,s
CHEMICAL
PHYSICS
LEXTERS
11 March 1983
_.
LASER-INDUCED FLUORESCENCE SPECTRA OF FREE-JET COOLED ORGANIC FREE RADICALS.
VINOXY, CYCLOPENTAMENYL, M. HEAVEN*,
AND BENZYL
L. DIMAURO and Terry A. MILLER
Bell Laboratories. Murra_vHill. New Jerse_v 07974. USA Received 3 January
1963
The organic free radicals, vinosy, cyclopentadienyl, and benzyl, have been produced in situ in a free jet cspansion escimer laser photolysis of a suitable precursor. Rotationally resolved. laser-induced tiuorcscence spectra. obtained downstream of the photolysis, show the gaseous radicals are cold (==10 1;).
1. Introduction Supersonic free jet expansions have contributed [1,2] enormously to recent spectroscopic progress. especially in two areas, the characterization of large stable molecules and many weakly bound van der Waals species. However, the advantages of super-cold, supersonic expansions have not been generally extended to short-lived, free radical species. The primary reason for lack of progress here is that free radicals, because of their extreme reactivity, must be prepared in situ in the expansion, a task involving some technical complexityRecently several reports have appeared concerning cold molecular ion spectra from jets. The initial reports [3-6] involved electron-bombardment-excited emission spectra of both small and large ions. More recently f7-9] laser-induced fluorescence spectra have been reported for ions and even ionic clusters. produced in expansions by both electron bombardment and multiphoton ionization. In many ways cations produced from neutral parents by simple electron ejection are iikely the easiest free radicals to produce ‘cold”_ assuming one has obtained a “cold” neutral precursor in the expansion, little or no rotational “warming” is likely in the ionization step [S ] _ Likewise the similar potential surface? often en* Present address: Department of Chemistry, Illinois Institute of Technology.
Chicago. Illinois 60616,
USA_
0 OOg-~614~83~0000-OOOO/~ 03.00 0 1983 North-Holland
by
countered for ions and their parent neutrals diminishes any vibrational excitation in the product ions. The situation is rather different for “chemical fragment” free radicals (ionic or neutral) produced by the rupture of a chemical bond. The energy liberated in the bond breaking will often find its way into the internal degrees of freedom of the fragment. The production of radicals with internal temperatures of several thousand kelvin from
Volurnc 95. nurnbcr 1.5
CHEMICAL
PHYSICS
radi4
in the high-pressure region. We have recently demonstrated [ 12,131 that efficient cooling of a free radical product can be obtained in a simple free jet espansion. In our initial espetiments we produced the simple diatornic free radical <‘S and Sl I by photolyzing. in the jet. BrCN and HIS rcspestively. For CN =4000 K of internal rotational energy was reduced to
2. Esperimental The free jet expansion apparatus has been described previously. so wc limit ourselves here to a discussion oi its principal aspects. The espansion is continuous. The iloz~k diameter is 0.25 mm. in these experiments 111~stagnation pressure of the Ar carrier gas behind 111eIW~AC wx 3-15 xtrn, with a chamber pressure of 20.1 --I Turr. A lightly focused excimer laser (Lumonies ShO-3) was used for the production of the free radicals vinoxy and cyclopentadienyl_ To produce b~~nzyl. the maximum power output of the excimer wx focus& to I spot with a 1 III lens. To produce Cii,CIiO. either ethyl or methyl vinyl ether was used as a precursor. and the excimer was run on the ArF transition. To produce C5 1-1s.cyclopentadiene was employ-ed as a precursor, and the excimer was run on the KrF transition. For C,H,CH,. toluene (and ben-
11 March 1983
LETTERS
zyl chloride to a lesser extent) was used as a precursor, and the excirner operated on the ArF transition. To obtain the best, cold spectra the excimer beam crossed the free jet expansion ~10 nozzle diameters downstream. The Nz pumped (Molectrcn UV24) dye laser (Molectron DLII) beam crossed the expansion =40 nozzle diameters downstream_ Some spectra, especially for vinoxy, have been taken at intermediate downstream locations_ The vinoxy spectrum can be seen to simplify continuously as it cools in the downstream expansion. To obtain rotational resolution, the dye laser had to be narrowed with an intracavity etalon. The laser frequency was scanned by varying the pressure of either N2 or SF, in the cavity from 0 to 1 atm. The specified linewidth of the laser in this configuration is ==0.03 cm- 1 and =0.05 cm-l when frequency doubled_ Calibration of the spectral scans was accomplished by simultaneously recording the laser-induced fluorescence spectrum of 1, or the optogalvanic spectrum of the U atom and referring to the respective atlases [ 14.15].
3. Results and discussion Laser-induced fluorescence spectra for the free radicals. vinoxy [ 16.171, cyclopentadienyl [IS] and benzyl [ 19-2 11, have recently been reported. In all cases the radicals were at room or elevated temperatures. and the spectra showed resolved, or partially resolved, vibrational structure_ However in no case was rotational resolution attempted due at least in part to the extreme spectral congestion present at moderate temperature for these species. Fig. 1 shows a scan over one vibrational band (010, following the notation of ref. [ 161) of the vinoxy free radical. Vinoxy is nearly a prolate symmetric top_ (Rotational constants calculated from the ab initio structure 1221 for vinoxy yield an asymmetry parameter [23] offi = -0.95 compared to -1 for a prolate symmetric
top.)
For
the purposes
of the present
dis-
cussion, we treat the vinosy spectrum as if it arose from a symmetric top. The strongest features in the spectrum can be assigned to P- and R-branch dA’ = 0 progressions for the lowest K = 0 level. Additional weak structure, which grows stronger at iess severe expansion conditions, probably arises from similar
CHEMICAL
Volume 95, number 43 YINOXY
ABBE.
RADICAL
AT
PHYSICS
11 March 1983
LETTERS
10K
R (2)
I
-L-__
_-__
29.568
!
29.573
29.578
cm-’
0. 29QWW.
29900.
29904. FREOUENCY
29909.
29912
cbW-1,
1. Frequency-doubled, pressure-tuned scan of etalonnarrowed laser over the 010 vibrational band of the B ‘A”% *A” electronic transition of the CH2CHO radical. The indicated rotational assignments are made on the assumption of a p3raIIeI transition of a near prolate symmetric top_ Use of this assignment yields a rotational temperature of z=10 K_
Fig. 2. Frequency-doubled. pressure-tuned scan of etalonnarrowed laser over the vibronic band traditionally assigned as the oripin of the x ‘A$ - a ‘E’; transition of CSHs_ This band exhibits a complicated 1 branch rotational structure and H detailed analysis is in proFess_
I-ig.
P and R progressions for levels with li’ > 0. Qualitatively similar rotational structure hasalso been resolved for the 000 and 100 vibronic transitions. Using the assignment given in fig. 1 yields a rotational temperature of ==lO K for the CHzCHO free radical. Fig. 2 shows a high-resolution spectrum for the cyclopentadienyl free radical. As can be seen in fig. 2, the rotational structure is not so simple as that of vinosy even thou& the species are of comparable molecular weight and cyclopentadienyl is nominally of much higher symmetry. The spectrum in fig. 2 was obtained using SF, as a pressurizing gas for the etalon scan. Somewhat better resolution is obtained when N1 is employed for this purpose. Probably the single most important reason for the increased complexity of the C5H5 rotational structure lies in the nature of the vibronic transition_ The band pictured in fig. 2 likely corresponds to the origin of the electronic transition x ?A: ++x ‘E’;. Such a transition will have [23] the more complicated 1 branch rotational structure, compared to the predominately ]I branch structure seen in fig. I_ While this 1 structure may slightly complicate a rotational analysis, it may well ultimately yield more information. Because of the complicated rotational structure, no precise determination of the rotational temperature of the C,H5 radical is yet available; however the overall appearance of the spectrum would suggest a temperature comparable to that determined for vinoxy.
Lower-resolution scans of the vibronic structure of the C5 H5 spectrum indicate substantial changes in intensity among various vibronic bands compared to earlier absorption [24,X] studies. For instance, pre-
vious authors have noted a doublet structure (~330 cm-l separation) of comparable intensity for several of the stronger bands, including that shown in high resolution in fig. 2. We find that the doublet companion is not of comparable intensity for this transition in our spectrum_ Such changes may reflect the lower temperature of the jet spectrum and/or predissociation in the excited state. The exact mechanism must await the results of more detailed espzriments. In fig. 3, we show a broad scan over the vibronic structure
of the benzyl free radical. This electronic
BENZYL
3008
I
a.
RADICAL
I
/
2227EL
22325_
223v5L FREOUENCY
KM-l>
22&s_
rrbs.
Fig. 3_ Broad scan over three vibronic band of the x *A2 z *Bz electronic transition of the &H&H2 radical. Coins from low to high frequency the three bands can be identified, respectively. as A’ _ A2, and 6ah, following the notation of ref. [27]_
349
Vohrmc 95, number 4.5
CHEWCAL PHYSICS LETTERS
11 BI.arch 1983
transition was identified several years ago [26--Z] as being between the ground 1 ?B1 state and two slose-
Acknowledgement
lying excited states, the I )A2 and the slightly higher 2 2 B2 _There are three strong vibronic bands shown in fig. 3 which are identified (following the notation of ref. [271) as -4’ and AZ. the first two A-type transitions (wherein the change in the dipole moment is along the A axis) and 6ai. the first B-type transition.
We are very grateful to M-L. Schilling and H-D. Roth for supplying us with the cyclopentadiene monomer, and the latter for helpful discussions_ We also thank J.R. McDonald and R.N. Zare for furnishing us preprints of their respective works.
There have of the the non-cooled of the A1 _ A?.
been two recently published figures laser-induced fluorescence spectrum of benzyl radical. The relative strengths and 63: bands are similar in those and our spectra. On the other hand the resolution between the bands is much improved in our cold spectrum. Likewise the contours of the bands have changed considerably. Only in the jet-cooled spectrum is there the clear doublet structure, with peaks of comparable magnitude. present in each band\i’e have also taken pressure tuned etalon scans across the A1 band in fig. 3. These showed clearly resolved rotational structure_ However this structure is very comples, owing to the asymmetric nature of the benzyl radical. However the resolution is clearly superior to that obtained for the emission spectra [I?.201
that were rnodelled iu the previous contour analysis [ZS]. The present spectra should allow a more refined determination of the rotational constants, A. B, C, thm
4.
was possible
with the contour
analysis.
Su111111ary
lxxr-iriduccd
Iluorescence
spectra
of three
of the
most important orgartic radicals - vinosy. cyclopendieuyl. and benzyl - have been obtained_ The gaseous free radicals are obtained with a very low internal tetupersture (=z IO I() by photolysis with an escimer laser in a supersonic free jet expansion. The low tetupcrsturc of the radical reduces sharply the complexity and couscsfion commonly present in the spectra of snch
large ~ttolecules. Kotational structure has been resolved for all three radicals. Although this structure IIKI~ in sonrc cases be quite comples, its observation promises significant new insights into the geometric structure, bonding. Jahn-Teller effects (C,Hs). and state-to-state dynamics of these radicals_
350
References [ 11 D.H. Levy. Ann. Rev. Phys. Chem. 31 (1950)
197_ [Z] A. Amirav, U. Even and _I_Jortner. Chem Phys. 5 1 (1980) 31. 13 ] A. Carrington and R. Tuckett. Chem. Phys. Letters 74 (1980) 19: K. Tuckett.Chem. Phys. 58 (1981) 151. ] T-A. Miller. B_R_ Zegarski, TJ. Sears and V-E. Bondybey. J_ Phys. Chem. 84 (1980) 3154. 1 B.M. DeKoven, D.H. Levy, H.H. Harris, B.R. ZeSarski and T-A. Miller, J. Chem. Phys. 74 (1981) 5659. ] D. Klapstein, S. Leutwyler and J.P. Maier. Chem. Phys. Letters 64 (1981) 534. [ 7) 51. Heaven. T-A. Miller and V.E. Bondybey, J. Chem. Phvs. 76 (1982) 3831. [S] T.A. Miller and V.E. Bondybey. Phil. Trans. Roy Sot. (London) A. !o be published_ 19) 1l.A. Johnson, J. Rostas and R.N. Zare. Chrm. Phys. Letters 92 (1982) 175.
1101 D.L. Monts.T.G.
Dietz, X1.A. Duncan and RX. Smalley,
Chem. Phys. 45 (1980) 133_ 1I1 J D.E. Powers. J.B. Hopkins and R.E. Smalley. J. Phys. Chem. 85 (1981) 2711. [ 12 1 .\I. tleaven, T-A. Miller and VI. Bondybey. Chem. t’hys. Letters 84 ( 198 1) 1. 113] $1. Heaven and T-A. Miller. Proceedings of the Conference on Lasers as Reactants and Probes in Chemistry (tioward Univ. Press). to be published. 1141 S. Cerstenkorn and P. Luc. .4tlas du spectre d’absorption de Is molecule d’iode (CNRS. Paris, 1978). [ 151 B.A. Palmer. R.A. Keller and R. Engleman Jr.. An Atlas of Uranium Emission Intensities in a Iiollow tiIhode Discharge, Los Atmos Scientific hbOratOry, LA-SZS141S (1980). [161 G_lnoueandH_Akimoto.J.Chem.Phys.74(1981)425. [ 17 J KXlrinermanns and A.C. Lunrz, J. Phys. Chem. 85 (19Sl) 1966. 118 J 1i.H. Nelson, L. Pasternak and J.R. McDonald, to be published. [ 191 D.&l. Brenner, G.P. Smith and R.N. Zare, J. Am. Chem. Sot. 98 (1976) 6707. [30] T. 0kamura.T.R. Charlton and B.A. Thrush. Chem. Phys. Letters 88 (1982) 369. 121 J H.H. Nelson and J-R. McDonald. J. Phys. Chem. 86 (1981) 1242.
Volume 95, number 4.5 [22]
CHEMKAL
hl. Dupuis, JJ. Wendoloski and W.A. Lester Jr., J. Chem. Pys. 76 (19Sz’) 488. 1231 G. Herzberg, Electronic spectra of polyatomic moiecules (Van Nostrand, Princeton, 1966). [24] G_ Porter and B. Ward, Proc. Roy. Sot. A303 (1968) 139.
PHYSlCS
LET=fERS
[25]
R. Engleman Jr. (1970) 964. 1261 G. Porter and B. [27] C_ Cossart-Majos 4006. 1281 C. Cossart-hfajos 1534.
11 March 1983 and D.A. Ramsay, Can. J_ Phys. 48 Ward, J. Chim. Phys. 61 (1964) 102. and S. Leach, J. Chem. Phys. 64 (1976) and S. Leach, J. Chem. Phys. 56 (1972)
CHEMICAL
PHYSICS
LEXTERS
11 March 1983
_.
LASER-INDUCED FLUORESCENCE SPECTRA OF FREE-JET COOLED ORGANIC FREE RADICALS.
VINOXY, CYCLOPENTAMENYL, M. HEAVEN*,
AND BENZYL
L. DIMAURO and Terry A. MILLER
Bell Laboratories. Murra_vHill. New Jerse_v 07974. USA Received 3 January
1963
The organic free radicals, vinosy, cyclopentadienyl, and benzyl, have been produced in situ in a free jet cspansion escimer laser photolysis of a suitable precursor. Rotationally resolved. laser-induced tiuorcscence spectra. obtained downstream of the photolysis, show the gaseous radicals are cold (==10 1;).
1. Introduction Supersonic free jet expansions have contributed [1,2] enormously to recent spectroscopic progress. especially in two areas, the characterization of large stable molecules and many weakly bound van der Waals species. However, the advantages of super-cold, supersonic expansions have not been generally extended to short-lived, free radical species. The primary reason for lack of progress here is that free radicals, because of their extreme reactivity, must be prepared in situ in the expansion, a task involving some technical complexityRecently several reports have appeared concerning cold molecular ion spectra from jets. The initial reports [3-6] involved electron-bombardment-excited emission spectra of both small and large ions. More recently f7-9] laser-induced fluorescence spectra have been reported for ions and even ionic clusters. produced in expansions by both electron bombardment and multiphoton ionization. In many ways cations produced from neutral parents by simple electron ejection are iikely the easiest free radicals to produce ‘cold”_ assuming one has obtained a “cold” neutral precursor in the expansion, little or no rotational “warming” is likely in the ionization step [S ] _ Likewise the similar potential surface? often en* Present address: Department of Chemistry, Illinois Institute of Technology.
Chicago. Illinois 60616,
USA_
0 OOg-~614~83~0000-OOOO/~ 03.00 0 1983 North-Holland
by
countered for ions and their parent neutrals diminishes any vibrational excitation in the product ions. The situation is rather different for “chemical fragment” free radicals (ionic or neutral) produced by the rupture of a chemical bond. The energy liberated in the bond breaking will often find its way into the internal degrees of freedom of the fragment. The production of radicals with internal temperatures of several thousand kelvin from
Volurnc 95. nurnbcr 1.5
CHEMICAL
PHYSICS
radi4
in the high-pressure region. We have recently demonstrated [ 12,131 that efficient cooling of a free radical product can be obtained in a simple free jet espansion. In our initial espetiments we produced the simple diatornic free radical <‘S and Sl I by photolyzing. in the jet. BrCN and HIS rcspestively. For CN =4000 K of internal rotational energy was reduced to
2. Esperimental The free jet expansion apparatus has been described previously. so wc limit ourselves here to a discussion oi its principal aspects. The espansion is continuous. The iloz~k diameter is 0.25 mm. in these experiments 111~stagnation pressure of the Ar carrier gas behind 111eIW~AC wx 3-15 xtrn, with a chamber pressure of 20.1 --I Turr. A lightly focused excimer laser (Lumonies ShO-3) was used for the production of the free radicals vinoxy and cyclopentadienyl_ To produce b~~nzyl. the maximum power output of the excimer wx focus& to I spot with a 1 III lens. To produce Cii,CIiO. either ethyl or methyl vinyl ether was used as a precursor. and the excimer was run on the ArF transition. To produce C5 1-1s.cyclopentadiene was employ-ed as a precursor, and the excimer was run on the KrF transition. For C,H,CH,. toluene (and ben-
11 March 1983
LETTERS
zyl chloride to a lesser extent) was used as a precursor, and the excirner operated on the ArF transition. To obtain the best, cold spectra the excimer beam crossed the free jet expansion ~10 nozzle diameters downstream. The Nz pumped (Molectrcn UV24) dye laser (Molectron DLII) beam crossed the expansion =40 nozzle diameters downstream_ Some spectra, especially for vinoxy, have been taken at intermediate downstream locations_ The vinoxy spectrum can be seen to simplify continuously as it cools in the downstream expansion. To obtain rotational resolution, the dye laser had to be narrowed with an intracavity etalon. The laser frequency was scanned by varying the pressure of either N2 or SF, in the cavity from 0 to 1 atm. The specified linewidth of the laser in this configuration is ==0.03 cm- 1 and =0.05 cm-l when frequency doubled_ Calibration of the spectral scans was accomplished by simultaneously recording the laser-induced fluorescence spectrum of 1, or the optogalvanic spectrum of the U atom and referring to the respective atlases [ 14.15].
3. Results and discussion Laser-induced fluorescence spectra for the free radicals. vinoxy [ 16.171, cyclopentadienyl [IS] and benzyl [ 19-2 11, have recently been reported. In all cases the radicals were at room or elevated temperatures. and the spectra showed resolved, or partially resolved, vibrational structure_ However in no case was rotational resolution attempted due at least in part to the extreme spectral congestion present at moderate temperature for these species. Fig. 1 shows a scan over one vibrational band (010, following the notation of ref. [ 161) of the vinoxy free radical. Vinoxy is nearly a prolate symmetric top_ (Rotational constants calculated from the ab initio structure 1221 for vinoxy yield an asymmetry parameter [23] offi = -0.95 compared to -1 for a prolate symmetric
top.)
For
the purposes
of the present
dis-
cussion, we treat the vinosy spectrum as if it arose from a symmetric top. The strongest features in the spectrum can be assigned to P- and R-branch dA’ = 0 progressions for the lowest K = 0 level. Additional weak structure, which grows stronger at iess severe expansion conditions, probably arises from similar
CHEMICAL
Volume 95, number 43 YINOXY
ABBE.
RADICAL
AT
PHYSICS
11 March 1983
LETTERS
10K
R (2)
I
-L-__
_-__
29.568
!
29.573
29.578
cm-’
0. 29QWW.
29900.
29904. FREOUENCY
29909.
29912
cbW-1,
1. Frequency-doubled, pressure-tuned scan of etalonnarrowed laser over the 010 vibrational band of the B ‘A”% *A” electronic transition of the CH2CHO radical. The indicated rotational assignments are made on the assumption of a p3raIIeI transition of a near prolate symmetric top_ Use of this assignment yields a rotational temperature of z=10 K_
Fig. 2. Frequency-doubled. pressure-tuned scan of etalonnarrowed laser over the vibronic band traditionally assigned as the oripin of the x ‘A$ - a ‘E’; transition of CSHs_ This band exhibits a complicated 1 branch rotational structure and H detailed analysis is in proFess_
I-ig.
P and R progressions for levels with li’ > 0. Qualitatively similar rotational structure hasalso been resolved for the 000 and 100 vibronic transitions. Using the assignment given in fig. 1 yields a rotational temperature of ==lO K for the CHzCHO free radical. Fig. 2 shows a high-resolution spectrum for the cyclopentadienyl free radical. As can be seen in fig. 2, the rotational structure is not so simple as that of vinosy even thou& the species are of comparable molecular weight and cyclopentadienyl is nominally of much higher symmetry. The spectrum in fig. 2 was obtained using SF, as a pressurizing gas for the etalon scan. Somewhat better resolution is obtained when N1 is employed for this purpose. Probably the single most important reason for the increased complexity of the C5H5 rotational structure lies in the nature of the vibronic transition_ The band pictured in fig. 2 likely corresponds to the origin of the electronic transition x ?A: ++x ‘E’;. Such a transition will have [23] the more complicated 1 branch rotational structure, compared to the predominately ]I branch structure seen in fig. I_ While this 1 structure may slightly complicate a rotational analysis, it may well ultimately yield more information. Because of the complicated rotational structure, no precise determination of the rotational temperature of the C,H5 radical is yet available; however the overall appearance of the spectrum would suggest a temperature comparable to that determined for vinoxy.
Lower-resolution scans of the vibronic structure of the C5 H5 spectrum indicate substantial changes in intensity among various vibronic bands compared to earlier absorption [24,X] studies. For instance, pre-
vious authors have noted a doublet structure (~330 cm-l separation) of comparable intensity for several of the stronger bands, including that shown in high resolution in fig. 2. We find that the doublet companion is not of comparable intensity for this transition in our spectrum_ Such changes may reflect the lower temperature of the jet spectrum and/or predissociation in the excited state. The exact mechanism must await the results of more detailed espzriments. In fig. 3, we show a broad scan over the vibronic structure
of the benzyl free radical. This electronic
BENZYL
3008
I
a.
RADICAL
I
/
2227EL
22325_
223v5L FREOUENCY
KM-l>
22&s_
rrbs.
Fig. 3_ Broad scan over three vibronic band of the x *A2 z *Bz electronic transition of the &H&H2 radical. Coins from low to high frequency the three bands can be identified, respectively. as A’ _ A2, and 6ah, following the notation of ref. [27]_
349
Vohrmc 95, number 4.5
CHEWCAL PHYSICS LETTERS
11 BI.arch 1983
transition was identified several years ago [26--Z] as being between the ground 1 ?B1 state and two slose-
Acknowledgement
lying excited states, the I )A2 and the slightly higher 2 2 B2 _There are three strong vibronic bands shown in fig. 3 which are identified (following the notation of ref. [271) as -4’ and AZ. the first two A-type transitions (wherein the change in the dipole moment is along the A axis) and 6ai. the first B-type transition.
We are very grateful to M-L. Schilling and H-D. Roth for supplying us with the cyclopentadiene monomer, and the latter for helpful discussions_ We also thank J.R. McDonald and R.N. Zare for furnishing us preprints of their respective works.
There have of the the non-cooled of the A1 _ A?.
been two recently published figures laser-induced fluorescence spectrum of benzyl radical. The relative strengths and 63: bands are similar in those and our spectra. On the other hand the resolution between the bands is much improved in our cold spectrum. Likewise the contours of the bands have changed considerably. Only in the jet-cooled spectrum is there the clear doublet structure, with peaks of comparable magnitude. present in each band\i’e have also taken pressure tuned etalon scans across the A1 band in fig. 3. These showed clearly resolved rotational structure_ However this structure is very comples, owing to the asymmetric nature of the benzyl radical. However the resolution is clearly superior to that obtained for the emission spectra [I?.201
that were rnodelled iu the previous contour analysis [ZS]. The present spectra should allow a more refined determination of the rotational constants, A. B, C, thm
4.
was possible
with the contour
analysis.
Su111111ary
lxxr-iriduccd
Iluorescence
spectra
of three
of the
most important orgartic radicals - vinosy. cyclopendieuyl. and benzyl - have been obtained_ The gaseous free radicals are obtained with a very low internal tetupersture (=z IO I() by photolysis with an escimer laser in a supersonic free jet expansion. The low tetupcrsturc of the radical reduces sharply the complexity and couscsfion commonly present in the spectra of snch
large ~ttolecules. Kotational structure has been resolved for all three radicals. Although this structure IIKI~ in sonrc cases be quite comples, its observation promises significant new insights into the geometric structure, bonding. Jahn-Teller effects (C,Hs). and state-to-state dynamics of these radicals_
350
References [ 11 D.H. Levy. Ann. Rev. Phys. Chem. 31 (1950)
197_ [Z] A. Amirav, U. Even and _I_Jortner. Chem Phys. 5 1 (1980) 31. 13 ] A. Carrington and R. Tuckett. Chem. Phys. Letters 74 (1980) 19: K. Tuckett.Chem. Phys. 58 (1981) 151. ] T-A. Miller. B_R_ Zegarski, TJ. Sears and V-E. Bondybey. J_ Phys. Chem. 84 (1980) 3154. 1 B.M. DeKoven, D.H. Levy, H.H. Harris, B.R. ZeSarski and T-A. Miller, J. Chem. Phys. 74 (1981) 5659. ] D. Klapstein, S. Leutwyler and J.P. Maier. Chem. Phys. Letters 64 (1981) 534. [ 7) 51. Heaven. T-A. Miller and V.E. Bondybey, J. Chem. Phvs. 76 (1982) 3831. [S] T.A. Miller and V.E. Bondybey. Phil. Trans. Roy Sot. (London) A. !o be published_ 19) 1l.A. Johnson, J. Rostas and R.N. Zare. Chrm. Phys. Letters 92 (1982) 175.
1101 D.L. Monts.T.G.
Dietz, X1.A. Duncan and RX. Smalley,
Chem. Phys. 45 (1980) 133_ 1I1 J D.E. Powers. J.B. Hopkins and R.E. Smalley. J. Phys. Chem. 85 (1981) 2711. [ 12 1 .\I. tleaven, T-A. Miller and VI. Bondybey. Chem. t’hys. Letters 84 ( 198 1) 1. 113] $1. Heaven and T-A. Miller. Proceedings of the Conference on Lasers as Reactants and Probes in Chemistry (tioward Univ. Press). to be published. 1141 S. Cerstenkorn and P. Luc. .4tlas du spectre d’absorption de Is molecule d’iode (CNRS. Paris, 1978). [ 151 B.A. Palmer. R.A. Keller and R. Engleman Jr.. An Atlas of Uranium Emission Intensities in a Iiollow tiIhode Discharge, Los Atmos Scientific hbOratOry, LA-SZS141S (1980). [161 G_lnoueandH_Akimoto.J.Chem.Phys.74(1981)425. [ 17 J KXlrinermanns and A.C. Lunrz, J. Phys. Chem. 85 (19Sl) 1966. 118 J 1i.H. Nelson, L. Pasternak and J.R. McDonald, to be published. [ 191 D.&l. Brenner, G.P. Smith and R.N. Zare, J. Am. Chem. Sot. 98 (1976) 6707. [30] T. 0kamura.T.R. Charlton and B.A. Thrush. Chem. Phys. Letters 88 (1982) 369. 121 J H.H. Nelson and J-R. McDonald. J. Phys. Chem. 86 (1981) 1242.
Volume 95, number 4.5 [22]
CHEMKAL
hl. Dupuis, JJ. Wendoloski and W.A. Lester Jr., J. Chem. Pys. 76 (19Sz’) 488. 1231 G. Herzberg, Electronic spectra of polyatomic moiecules (Van Nostrand, Princeton, 1966). [24] G_ Porter and B. Ward, Proc. Roy. Sot. A303 (1968) 139.
PHYSlCS
LET=fERS
[25]
R. Engleman Jr. (1970) 964. 1261 G. Porter and B. [27] C_ Cossart-Majos 4006. 1281 C. Cossart-hfajos 1534.
11 March 1983 and D.A. Ramsay, Can. J_ Phys. 48 Ward, J. Chim. Phys. 61 (1964) 102. and S. Leach, J. Chem. Phys. 64 (1976) and S. Leach, J. Chem. Phys. 56 (1972)