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Collisional coupling of N2(B3Πg) and N2(W3Δu) states studied by laser-induced fluorescence
Volume
COLLISIONAL STUDIED
Received
COUPLING
OF N,(B
BY LASER-INDUCED
N- SADEGHI* Department
15 January 1981
CHEMICAL PHYSICS LETTERS
11, number 2
311g) AND N,(W 3h)
STATES
FLUORESCENCE
and D.W. SETSER
of Chemrstty.
27 September
Kansas 1980;
State
m foal
Umversrty.
Manhattan,
form 13 October
Kansas
66506.
USA
1980
The N1(B ‘llg) state aas produced by laser eucltauon cf N2(A 3 X:) m Ne or Ar tamer gas m a flowmg aftcrglo\v apparatus. AU of the vibrattond levels rrutxdly populated by the laser, U’ = 3- 10, showed double evponen&al relaxation The lrutlal fast decay IS associated -i%xthcollm~ml coupling of the N=(B, u’) level to adjacent N20V 3&,) vtbrattonal levels The slow decay represents the overall decay of these coupled levels, which generates lower vtbrational level,, of Nz(B 3 fig)
1. lntrodsction The N,(B 3lI ) state 1s commonly observed in electncal drscharges 711, m the rutrogen afterglow [2,3], from excrtatton transfer reactrons wrth N, and N20 [4--61 and photodrssocratron of N70 [7]. Frequently the N?(B) vrbratronal drstnb&ion 7s of mterest. We report laser-mduced fluorescence data for the u = 3- 10 vrbrational levels wtuch show that vrbratronal relaxation of N-JB) 1s severe rn 1 Torr of Ar and srgniticant even in 1 Torr of Ne. Tins vlbratronal relaxatron occurs via rapid colhsional coupling of specific N2(B 311g) and N2(W 3&) vrbratronal levets. Several of the N2(B) vrbrational levels are so strongly coupled to N,CW) lelrels that the population IS equrhbrated even at modest pressures of Ne or Ar. The pattern of our data and the energy separations between the u = 0 and 1 levels of N,(B 3flg) and N2(W 3&,) suggest that under most expernnental condrtions (except for truly collision-free situations) the chemistry of both N?(B) and N?_(W) must be considered whenever N,(B, u = 0- 10) IS observed in a buffer gas. Thrs conclusion may slgnrficantly affect existing mterpretatrons which have been made assuming that only N2(B) was present. At rhe very least, many claims about the “in&al” N2(B) vibrational populations from a given excitation source * Permanent address: Laboratoue de Spectrometne Physique, UniversitC de Grenoble, 38041 Grenoble, France.
need revrsron because of the very fast vrbrational relaxation. In addition to the coupltng between N,(B 31Tg) and N2(W 3&), there 1s some evrdence for a couphng to yet a tturd state (see fig. 1 for the possrbdrties) but we cannot yet identify the thrrd state. The colhsional coupling of N2(B 3 ll ) and N2(W 3&,) 1s the second example of an electromca b y excited dratomrc molecule that etirbrts fast vrbratronal relaxahon by a collrsron-mduced intersystem crossmg mechanlsm mvolvmg an electronic state of different symmetry. The first example was relaxation of N$(A 2nu) via crossmg to N$(X 2ZZ3 m He [S], Mdler, Bondybey and co-workers also have observed relaxatron by a
sin-mar mechanism for CN(A’fI) and CO+(A2fI), but for these cases strong rotational perturbations between the CN(A 2 ff) and CN(X 2ZZ) or CO+(A) and CO+(X ‘Z) states can be mvoked to explam the collwon-Induced crossmg. or N,(B)
This argument
IS not applicable
to the s_(A)
cases, and explanation of the collisronal coup%ng IS not yet avadable. This study uttlrzed the N,(B-A) fluorescence followmg the laser excitation from NZ(A 3YZi), which was produced by excrtation transfer from A.r(3P2) or Xe(3PZ) to N3(X) m a flowing afterglow apparatus using Ne or Ar earner gas. The experimental data are laser-induced fluorescence spectra of N,(B, u’) and decay plots for individual vrbrational levels followmg laser excitation from NZ(A). This paper presents the general qualitative conclusions. A complete analysrs,
CHEMICAL PHYSICS LETTERS
Volume 77, number 2
Brewster-angle windows were added to the N2(A) reactor sectlon. The excltatlon transfer reactions of
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15 January 1981
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Ar(3P0,-J and Xe(3P2) + N?_(X) were used to generate the N,(A) molecules rn erther Ar or Ne carrier. We found-that adltlon of Ar or Xe to the Ne flow before the gases passed through the hollow cathode discharge provided adequate Ar* or Xe” metastable atoms for the eucltation-transfer reaction. The generatlon of Xe(3P2) in Ar earner 1s easily accomphshed by addmg small amounts of Xe to the Ar flow [4,12]. The Ne and Ar tank gases were purified by passmg the gas flow through cooled molecular sieve traps. (Care should be everclsed wth Ar smce hquld argon WIII condense m a cooled sieve trap If the pressure IS too Hugh.) The operatmg pressure range of our flowing afterglow apparatus IS l-10 Torr. Heidner et al. [ 151 first showed that N2(B) could be observed by laser-Induced fluorescence m a N,(A) flowing afterglow system. We employed a CMX4 Chromatl?c
flash pumped
dye laser Lvlth rhodamme-
590 and oxazine-720 to generate N-,(B, u’= 4-I 0) and N,(B, u’ = 3), respectively. The-u’ = 4-I 0 levels
Frg 1. Listing of energy levels for N2 in the 59000-76000 cm-l energy range, deduced from ref. [ 181. The strongly coupled N*(B) and N2flV) levels are shown by double arrows. Smgle arrows show
one-way coupling
were populated by laser excitation from the N2(A) u = 0 to 6 levels, respectively, by tuning the laser to the Av = 4 system band heads. The v’= 3 level was populated by tunmg the laser to the O-3 band head. To our surprise, m the laser excrtation regon, located about 20 ms after the mixing region of the Inert-gas metastable atoms and nitrogen, we had enough N2(A) III the wbratlonal levels up to 6 to be able to populate N2(B) up to v’ = 10. On the contrary the population of NZ(A, u = 7) IS very low and pumping of the N,(B, u’= I I) was not possible. InspectIon of fig. 1 suggests
givmg mdivlduai decay constants and product state formatIon rate constants requires numerical data fittmg
that this may be a consequence of the colhsional coupling of N,(B, u’= 0) and the N,(A, u=7) level
and WIU be reported in the future [ 11) _ For convemence the N,(B) vibrational level initially produced by the laser $lse wrfl be mdlcated as u’, and u” denotes the N2(W) level(s) dlrectiy coupled to the u’ level by colhs~ons; see fig. 1 for a presentation of the energy levels.
followed by radiative decay of N,(B, v’= 0). For laser excitation from NZ(A, u 2 3), the Xe(3P2) + N2 reaction IS the better source because it generates a higher N,(A 3Zi) vlbratlonal distribution. The N2(B-A) fluorescence was observed at wavelengths longer than the laser pump wavelength with a 1 m McPherson monochromator fitted with an RCA C3 1034 photomultlpher tube. The signal was fed to a pre-amphfier and then to a Blomatlon transient dlgltlzer, whch was interfaced to a PDP-11 computer for signal aver-
2. Experimental
method
The N,(A 3Zz) molecules were generated III the small flowmg afterglow apparatus [I2 J that has been
used previously for stuches of N?_(A) reactlons [13,14] in our laboratory. In the present work, baffle arms
agmg. To record a decay plot required typically 5000 pulses. Smce the fluorescence was vlewed with a monochromator, laser scattered light was not a serious 305
Volume 77, number 2
CHEMICAL
PHYSICS LETTERS
problem. To record a totai emission spectrum the drgitizer and computer were replaced wrth a boxcar averager and the signal was recorded with a 5 to 50~s gate while the monochromator was scanrung. The only unusual feature of the present euperunents, relative to previous N*(A) studies by the flowmg afterglow techmque, is the requirement of low N, (or Xe) flows. Results from the Xe*(3P,) to N2(X) excitatron transfer reaction [ 161 showed that rf partral pressures greater than 0.3 mTorr for xenon and 3 mTorr for N, were used, relaxation of N-,(B, u’) from colhsrons with N2 or Xe were observed. G the present work the N, and Xe flows were reduced by a factor of 3 belorv the range where a srgnrficant effect could be observed_ Relaxation by colbstons of N2 or Xe wrth Nz(B) de& nitely is neghgrbie u-rthe present expenments. The low NZ concentratron also assists III preserving popuiatron to the hrgher u levels of N,(A). The most serrous hmrtatron to the present stud; is the long pulse length (1 MS) of the Chromatrx laser. Although the typrcal radratrve hfetunes of N,(B, u’) are =6~.~s [17], the experimental hfetunes were consrderably shorter. For the N,(B) levels that are strongly coupled to N2W), the uutial component of the decay curve IS too fast to be observed clearly and merges w~tb the N-,(B) excrtatron pulse.
15 January 1981
Fig 2 Fluorescence spectra of Nz(B) followng
laser evctiation
Torr of AL (A) Spectrum collected wrth boxcar averager gate set at 5 ,us on 100 mV scale. (B) Spectrum N2(B, u’= 5) III 2
3. Experimental
results
A N2(B-A) fluorescence spectrum following laser excrtation of N,(B, u’= 5) m 2 Torr of a-gon IS shown in fig. 2. Vibrational relaxation clearly is extensive even at modest pressures of Ar and Ne, wrth srgmficant populations bemg observed in vrbrational levels below u’= 5. In order to understand the mechamsm of vibrational relaxation, time resolved measurements for the decay of the initrally excited and for-matron of rhe lower levels of N?_(B) were done for Ar and Ne pressures ranging from 1 S-10 Torr. Some decay plots for u’ = 5 and 7 are shown in figs. 3 and 4, respectively. The Ne data for u’ = 5 show an rmtial fast decay followed by a second slower decay for the whole pressure range. The rate constant for the mitral decay mcreases hnearly with Ne pressure; however, the slower decay rate is virtually invariant wth Ne pressure. We interpret the irutral decay process as collisional couplmg of N,(B, u’= 5) and N,(\V, u” = 5 and 6) levels. Although the second process 1s vu-tuahy 306
collected wth boxcar averager gate set at 50~s on 500 mV scale. (C) Spectrum collected with boxcar avetager gate set at 50 ps but delayed 10 MSafter the laser pulses on 50 mV scale.
Fig. 3. Decay plots for Nz(B, u’= 5) in Ne (-) and Ar(- - -.)_ The nsmg parts of the pulses for the 5.1 Torr (Ne) and 0.9 Torr (Ar) are identxal and the dotted curve IS not shown. The Ne curves are double exponential wrth the first component lasting from l-4 c.rsafter the peak of the laser pulse.
VOlumc
77. number
2
CHIXICAL
PHYSICS
LETTERS
15 January
1981
For u’ =6,7 and 8 two-component decay could be barely observed at the lowest Ne pressures, 1 4 Torr. At higher Ne pressure or with Ar earner
component
gas the fast
merges wnh the laser excitation
pulse and decay IS observed- see fig 4. This slower decay component corresponds to the loss of populatron from the coupled N,(B, u’) and the N3(W, u’+ I) levels, which have nearly the same energy as shown m fig. 1. The rate constant of the slow process seems to be independent of Ne pressure but has a weak dependence on Ar pressure which unphes that the main contnbutron to the loss of the coupled (B,u’) and (W, u”) levels IS radiative decay (both W-B and B-A). The B, u’ = 10 level was studied only in Ar; the decay curves seemed to resemble those of u’= 9 A twocomponent decay was observable at low pressure, but at high pressure only a rather small amphtude long term component could be identified. The above analysis adequately evplams the decay of the uutmlly generated N,(B, u’) level, but formatron of the N2(B, u’- 1 and u’- 3) levels, whtch corresponds to apparent vibratronal relaxation, strll remams to be discussed. The mechanism for formatlon of u’- 1 and u’- 2 1s colhsional couplmg of the N2(W 3A,, , u”) level and the lower levels of N,(B). In addttron \.ve expect that the radrattve decay of N,(W,u”) to !ow levels of N?(B) IS Important [ 181. However, observation of the u = 0 and 1 levels 1s quite drfficult and thrs suppositron has not yet been proven The time dependence of fluoonly single exponentml
0
_- ____ 10
T!%,USE??
40
5
kg. 4. Decay plots for Nz(B, u’= 7) m Ne(-_) and Ar(- - -). The three Ne decay plots have smgle exponential decay and the same decay constant, the constant IS considerably larger m Ar.
mvarlant
wrth Ne pressure, it IS pressure dependent m Ar. The slow decay step mcludes both radratrve [to low N?(B, u)] levels and colhsional transfer, at least m Ar. Although formulatron of N#,u”= 5) probably is a product from the hutrat decay step, the large energy gap prevents /=5 from being mvolved in the slow decay of
the coupled N2(B. u’=5) and N3(W,u”=6) levels The N,(B, u’=4) level shows sirmlarbehavior to u’=S, except that the second decay component IS extremely weakand the overall decay 1s domrnated by the first component_ Thrs IS a consequence of u’ = 4 being nearly midway between the u”= 4 and 5 levels of N2(W). Formation of N2(W, u”= 5) from N,(B, u’=4) requires 585 cm-t and transfer back from fi,(W, u”=4) to N,(B,u’=4) is endothermic by 800 cm-t. The decay of u’ = 3 IS very smular to u’=4. The u’ = 9 level also follows the same scheme as u’ = 3 and 4, but since N,(W, u” = 10) IS only 490 cm-1 below u’=9, the amplitude of the second decay component is somewhat more important than for v’=3 and 4. Extrapolatron of the fast decay rate constant for u’ = 3-5 to zero Ne pressures grven mtercepts that are consistent wtth the N,(B) radiative hfetlmes [ 17 ] _ The decay of u’= 5 m A: &o IS shown u-r fig 3. Wrth argon the untial decay process IS so fast that It is virtually mdlstingurshable from the excrtatron pulse for intermedrate pressures and only the second slower component IS clearly evrdent. Thrs also IS the case for u’ = 3,4 and 9.
kg. 5. nuorescence levels after Nz(B,
curves in neon earner u+ 1) laser eucltatnon
gas
Nz(B.u) sharp spike
for some
The inltd
IS fluorescence from the laser dye that is observed even though the enuwon was wewed through the monochromntor.
307
Volume
77. number
CHEhfICAL
2
PHYSICS LETTERS
rescence from u = 4 (after u’= 5 excitation) and u = 6 (from u’=7 excrtation) is shown in fig. 5. The rrse time of the fluorescence and the tune for maxrrnum fluorescence mtensty is consistent with Nz(B, u’- 1) being associated with the slow N7(B, u’) decay component, i.e., the formation of N,(B, u’- 1) is too slow to be a dnect process from N2(B, u’). Analysts of the u’- 1 decay curves is complicated by the raprd coupling of N,(B,
15 January 1981
Acknowledgement I?us work was supported by the US Department of Energy and by the US Office of Naval Research.
We thank Professor S. Rosenwaks for communicating their laser-mduced fluorescence studies of N2(B) to us prior to publication.
u’- I) and the N2(W, u”) level. However. the exponenwmch probably tral decay of N,(B, u’- 1) fluorescence, corresponds to the formatlon process, seemed to be pressure dependent. FmaI resolution of the radiative and colhsional steps for the N2(W, u”) levels swats more complete analysrs and also have expenmental data.
References A L Lyutyb L-D. hfel’N!kova and N L. Sokolova. Soviet Phys. .J. 18 (1975) 46. r21 A.N. Wright-and d A. Wmkler. Active rutiogen (Academic Press. New York. 19681. [31 M F. Golde and i A. Inrush, Rept. Progr. Phys 36 (1973) 1285 r41 D.W. Setser and D H. Stedman, J. Chem. Phys. 52 (1970) [l]
3957.
4. Conclusions
151 L.A. Gundel, D.W. Setser. hf.A A. Clyne, J A. Covon and The present laser-induced
fluorescence
studies of
N,(B 3rIg, u’= 3-10) have demons!rated rapid, twostep, vrbratronal relaxation revolving a second electronrc state of N,. The second state IS identified as N,(W 3A,,) on the basis of the pattern of relaxatron of the individual N,(B, u’) levels. If thrs pattern of relaxatron also holds for the u’=O and 1 levels of N,(BsR ), the low levels also wrll be strongly coupled to N2(W 4 Au)_ These low N2(W) levels have very long raciiatrve lifetimes [18] and the N*(W) state may act as a reservoir state for the u =O and 1 levels of N2(B) in many expenmental situatrons. Indirect evidence suggests that the low levels of B and Ware collisronahy coupled to the A(3ZL) manifold of levels. The present work supports the general correlation suggested by Miller and Bondybey [8,91 that the ease of colhsronal crossing between states depends on both the FranckCondon factors and the energy defect. Further expenmental work is in progress.
308
[6]
W. NIP, J Chem. Phys. 64 (1976) 4390. T-D Nguyen, N. Sadeghl and J.C. Pebay-Peyroula,
Chem.
Phys. Letters 29 (1974) 242. [ 7 J G. Black, R L. Sharpless, T G Slanger and D C. Lorents, J. Chem. Phys. 62 (1975) 4266,4274 D-H. Katayama, T-A. hfiller and V E Bondybey, J. Chem Phys 72 (1980) 5469. [ 91 V-E. Bondybey and T-A. h¶iUer, J. Chem. Phys. 69 (1978) 3597. [lo] D-H. Katayama, T.A. Biller and V-E Bondebey, J. Chem Phys. 71 (1979) 1662. I1 11 N. Sadeghi and D-W- Setser, I. Chem Phys., to be published. [8]
[ 121 J.H Kolts and D-W. Setser, III: Reactive mtermetites 111 the gas phase, ed D.W. Setser (Academic Press, New York, 1979). [13] I. Nadler, D.W. Setser and S. Rosenwaks, Chem Phys. Letters 72 (1980) 536. [ 141 W-C. Clark and D W. Setser, J. Phys. Chem 84 (1980), to be published. [15] R.F. Heldner LH, D.G. Sutton and S.H Suchard, Chem Phys Letters 37 (1976) 243. [ 161 N. Sadeghi and D.W. Setser, Chem. Phys. Letters (1980), to be submitted for pubhcation. [17] M. Jeunehomme, J. Chem Phys. 45 (1966) i80.5. [ 181 A. Lofthus and P-H. Krupeme, J. Phys. Chem. Ref. Data 6 (1977) 113.
COLLISIONAL STUDIED
Received
COUPLING
OF N,(B
BY LASER-INDUCED
N- SADEGHI* Department
15 January 1981
CHEMICAL PHYSICS LETTERS
11, number 2
311g) AND N,(W 3h)
STATES
FLUORESCENCE
and D.W. SETSER
of Chemrstty.
27 September
Kansas 1980;
State
m foal
Umversrty.
Manhattan,
form 13 October
Kansas
66506.
USA
1980
The N1(B ‘llg) state aas produced by laser eucltauon cf N2(A 3 X:) m Ne or Ar tamer gas m a flowmg aftcrglo\v apparatus. AU of the vibrattond levels rrutxdly populated by the laser, U’ = 3- 10, showed double evponen&al relaxation The lrutlal fast decay IS associated -i%xthcollm~ml coupling of the N=(B, u’) level to adjacent N20V 3&,) vtbrattonal levels The slow decay represents the overall decay of these coupled levels, which generates lower vtbrational level,, of Nz(B 3 fig)
1. lntrodsction The N,(B 3lI ) state 1s commonly observed in electncal drscharges 711, m the rutrogen afterglow [2,3], from excrtatton transfer reactrons wrth N, and N20 [4--61 and photodrssocratron of N70 [7]. Frequently the N?(B) vrbratronal drstnb&ion 7s of mterest. We report laser-mduced fluorescence data for the u = 3- 10 vrbrational levels wtuch show that vrbratronal relaxation of N-JB) 1s severe rn 1 Torr of Ar and srgniticant even in 1 Torr of Ne. Tins vlbratronal relaxatron occurs via rapid colhsional coupling of specific N2(B 311g) and N2(W 3&) vrbratronal levets. Several of the N2(B) vrbrational levels are so strongly coupled to N,CW) lelrels that the population IS equrhbrated even at modest pressures of Ne or Ar. The pattern of our data and the energy separations between the u = 0 and 1 levels of N,(B 3flg) and N2(W 3&,) suggest that under most expernnental condrtions (except for truly collision-free situations) the chemistry of both N?(B) and N?_(W) must be considered whenever N,(B, u = 0- 10) IS observed in a buffer gas. Thrs conclusion may slgnrficantly affect existing mterpretatrons which have been made assuming that only N2(B) was present. At rhe very least, many claims about the “in&al” N2(B) vibrational populations from a given excitation source * Permanent address: Laboratoue de Spectrometne Physique, UniversitC de Grenoble, 38041 Grenoble, France.
need revrsron because of the very fast vrbrational relaxation. In addition to the coupltng between N,(B 31Tg) and N2(W 3&), there 1s some evrdence for a couphng to yet a tturd state (see fig. 1 for the possrbdrties) but we cannot yet identify the thrrd state. The colhsional coupling of N2(B 3 ll ) and N2(W 3&,) 1s the second example of an electromca b y excited dratomrc molecule that etirbrts fast vrbratronal relaxahon by a collrsron-mduced intersystem crossmg mechanlsm mvolvmg an electronic state of different symmetry. The first example was relaxation of N$(A 2nu) via crossmg to N$(X 2ZZ3 m He [S], Mdler, Bondybey and co-workers also have observed relaxatron by a
sin-mar mechanism for CN(A’fI) and CO+(A2fI), but for these cases strong rotational perturbations between the CN(A 2 ff) and CN(X 2ZZ) or CO+(A) and CO+(X ‘Z) states can be mvoked to explam the collwon-Induced crossmg. or N,(B)
This argument
IS not applicable
to the s_(A)
cases, and explanation of the collisronal coup%ng IS not yet avadable. This study uttlrzed the N,(B-A) fluorescence followmg the laser excitation from NZ(A 3YZi), which was produced by excrtation transfer from A.r(3P2) or Xe(3PZ) to N3(X) m a flowing afterglow apparatus using Ne or Ar earner gas. The experimental data are laser-induced fluorescence spectra of N,(B, u’) and decay plots for individual vrbrational levels followmg laser excitation from NZ(A). This paper presents the general qualitative conclusions. A complete analysrs,
CHEMICAL PHYSICS LETTERS
Volume 77, number 2
Brewster-angle windows were added to the N2(A) reactor sectlon. The excltatlon transfer reactions of
\nth
103 ml-~
75-
,,_S’-
\-a
ll-
/
lo/
22-
2019-
6-%qj_
m-
/
7-
23-
21-
g-,\
=8-
17-
706-+7-
/
766-
/
1514-
“-:
13-
4_/;\
653-
15 January 1981
12-
/--
ll-
\-
Ar(3P0,-J and Xe(3P2) + N?_(X) were used to generate the N,(A) molecules rn erther Ar or Ne carrier. We found-that adltlon of Ar or Xe to the Ne flow before the gases passed through the hollow cathode discharge provided adequate Ar* or Xe” metastable atoms for the eucltation-transfer reaction. The generatlon of Xe(3P2) in Ar earner 1s easily accomphshed by addmg small amounts of Xe to the Ar flow [4,12]. The Ne and Ar tank gases were purified by passmg the gas flow through cooled molecular sieve traps. (Care should be everclsed wth Ar smce hquld argon WIII condense m a cooled sieve trap If the pressure IS too Hugh.) The operatmg pressure range of our flowing afterglow apparatus IS l-10 Torr. Heidner et al. [ 151 first showed that N2(B) could be observed by laser-Induced fluorescence m a N,(A) flowing afterglow system. We employed a CMX4 Chromatl?c
flash pumped
dye laser Lvlth rhodamme-
590 and oxazine-720 to generate N-,(B, u’= 4-I 0) and N,(B, u’ = 3), respectively. The-u’ = 4-I 0 levels
Frg 1. Listing of energy levels for N2 in the 59000-76000 cm-l energy range, deduced from ref. [ 181. The strongly coupled N*(B) and N2flV) levels are shown by double arrows. Smgle arrows show
one-way coupling
were populated by laser excitation from the N2(A) u = 0 to 6 levels, respectively, by tuning the laser to the Av = 4 system band heads. The v’= 3 level was populated by tunmg the laser to the O-3 band head. To our surprise, m the laser excrtation regon, located about 20 ms after the mixing region of the Inert-gas metastable atoms and nitrogen, we had enough N2(A) III the wbratlonal levels up to 6 to be able to populate N2(B) up to v’ = 10. On the contrary the population of NZ(A, u = 7) IS very low and pumping of the N,(B, u’= I I) was not possible. InspectIon of fig. 1 suggests
givmg mdivlduai decay constants and product state formatIon rate constants requires numerical data fittmg
that this may be a consequence of the colhsional coupling of N,(B, u’= 0) and the N,(A, u=7) level
and WIU be reported in the future [ 11) _ For convemence the N,(B) vibrational level initially produced by the laser $lse wrfl be mdlcated as u’, and u” denotes the N2(W) level(s) dlrectiy coupled to the u’ level by colhs~ons; see fig. 1 for a presentation of the energy levels.
followed by radiative decay of N,(B, v’= 0). For laser excitation from NZ(A, u 2 3), the Xe(3P2) + N2 reaction IS the better source because it generates a higher N,(A 3Zi) vlbratlonal distribution. The N2(B-A) fluorescence was observed at wavelengths longer than the laser pump wavelength with a 1 m McPherson monochromator fitted with an RCA C3 1034 photomultlpher tube. The signal was fed to a pre-amphfier and then to a Blomatlon transient dlgltlzer, whch was interfaced to a PDP-11 computer for signal aver-
2. Experimental
method
The N,(A 3Zz) molecules were generated III the small flowmg afterglow apparatus [I2 J that has been
used previously for stuches of N?_(A) reactlons [13,14] in our laboratory. In the present work, baffle arms
agmg. To record a decay plot required typically 5000 pulses. Smce the fluorescence was vlewed with a monochromator, laser scattered light was not a serious 305
Volume 77, number 2
CHEMICAL
PHYSICS LETTERS
problem. To record a totai emission spectrum the drgitizer and computer were replaced wrth a boxcar averager and the signal was recorded with a 5 to 50~s gate while the monochromator was scanrung. The only unusual feature of the present euperunents, relative to previous N*(A) studies by the flowmg afterglow techmque, is the requirement of low N, (or Xe) flows. Results from the Xe*(3P,) to N2(X) excitatron transfer reaction [ 161 showed that rf partral pressures greater than 0.3 mTorr for xenon and 3 mTorr for N, were used, relaxation of N-,(B, u’) from colhsrons with N2 or Xe were observed. G the present work the N, and Xe flows were reduced by a factor of 3 belorv the range where a srgnrficant effect could be observed_ Relaxation by colbstons of N2 or Xe wrth Nz(B) de& nitely is neghgrbie u-rthe present expenments. The low NZ concentratron also assists III preserving popuiatron to the hrgher u levels of N,(A). The most serrous hmrtatron to the present stud; is the long pulse length (1 MS) of the Chromatrx laser. Although the typrcal radratrve hfetunes of N,(B, u’) are =6~.~s [17], the experimental hfetunes were consrderably shorter. For the N,(B) levels that are strongly coupled to N2W), the uutial component of the decay curve IS too fast to be observed clearly and merges w~tb the N-,(B) excrtatron pulse.
15 January 1981
Fig 2 Fluorescence spectra of Nz(B) followng
laser evctiation
Torr of AL (A) Spectrum collected wrth boxcar averager gate set at 5 ,us on 100 mV scale. (B) Spectrum N2(B, u’= 5) III 2
3. Experimental
results
A N2(B-A) fluorescence spectrum following laser excrtation of N,(B, u’= 5) m 2 Torr of a-gon IS shown in fig. 2. Vibrational relaxation clearly is extensive even at modest pressures of Ar and Ne, wrth srgmficant populations bemg observed in vrbrational levels below u’= 5. In order to understand the mechamsm of vibrational relaxation, time resolved measurements for the decay of the initrally excited and for-matron of rhe lower levels of N?_(B) were done for Ar and Ne pressures ranging from 1 S-10 Torr. Some decay plots for u’ = 5 and 7 are shown in figs. 3 and 4, respectively. The Ne data for u’ = 5 show an rmtial fast decay followed by a second slower decay for the whole pressure range. The rate constant for the mitral decay mcreases hnearly with Ne pressure; however, the slower decay rate is virtually invariant wth Ne pressure. We interpret the irutral decay process as collisional couplmg of N,(B, u’= 5) and N,(\V, u” = 5 and 6) levels. Although the second process 1s vu-tuahy 306
collected wth boxcar averager gate set at 50~s on 500 mV scale. (C) Spectrum collected with boxcar avetager gate set at 50 ps but delayed 10 MSafter the laser pulses on 50 mV scale.
Fig. 3. Decay plots for Nz(B, u’= 5) in Ne (-) and Ar(- - -.)_ The nsmg parts of the pulses for the 5.1 Torr (Ne) and 0.9 Torr (Ar) are identxal and the dotted curve IS not shown. The Ne curves are double exponential wrth the first component lasting from l-4 c.rsafter the peak of the laser pulse.
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For u’ =6,7 and 8 two-component decay could be barely observed at the lowest Ne pressures, 1 4 Torr. At higher Ne pressure or with Ar earner
component
gas the fast
merges wnh the laser excitation
pulse and decay IS observed- see fig 4. This slower decay component corresponds to the loss of populatron from the coupled N,(B, u’) and the N3(W, u’+ I) levels, which have nearly the same energy as shown m fig. 1. The rate constant of the slow process seems to be independent of Ne pressure but has a weak dependence on Ar pressure which unphes that the main contnbutron to the loss of the coupled (B,u’) and (W, u”) levels IS radiative decay (both W-B and B-A). The B, u’ = 10 level was studied only in Ar; the decay curves seemed to resemble those of u’= 9 A twocomponent decay was observable at low pressure, but at high pressure only a rather small amphtude long term component could be identified. The above analysis adequately evplams the decay of the uutmlly generated N,(B, u’) level, but formatron of the N2(B, u’- 1 and u’- 3) levels, whtch corresponds to apparent vibratronal relaxation, strll remams to be discussed. The mechanism for formatlon of u’- 1 and u’- 2 1s colhsional couplmg of the N2(W 3A,, , u”) level and the lower levels of N,(B). In addttron \.ve expect that the radrattve decay of N,(W,u”) to !ow levels of N?(B) IS Important [ 181. However, observation of the u = 0 and 1 levels 1s quite drfficult and thrs suppositron has not yet been proven The time dependence of fluoonly single exponentml
0
_- ____ 10
T!%,USE??
40
5
kg. 4. Decay plots for Nz(B, u’= 7) m Ne(-_) and Ar(- - -). The three Ne decay plots have smgle exponential decay and the same decay constant, the constant IS considerably larger m Ar.
mvarlant
wrth Ne pressure, it IS pressure dependent m Ar. The slow decay step mcludes both radratrve [to low N?(B, u)] levels and colhsional transfer, at least m Ar. Although formulatron of N#,u”= 5) probably is a product from the hutrat decay step, the large energy gap prevents /=5 from being mvolved in the slow decay of
the coupled N2(B. u’=5) and N3(W,u”=6) levels The N,(B, u’=4) level shows sirmlarbehavior to u’=S, except that the second decay component IS extremely weakand the overall decay 1s domrnated by the first component_ Thrs IS a consequence of u’ = 4 being nearly midway between the u”= 4 and 5 levels of N2(W). Formation of N2(W, u”= 5) from N,(B, u’=4) requires 585 cm-t and transfer back from fi,(W, u”=4) to N,(B,u’=4) is endothermic by 800 cm-t. The decay of u’ = 3 IS very smular to u’=4. The u’ = 9 level also follows the same scheme as u’ = 3 and 4, but since N,(W, u” = 10) IS only 490 cm-1 below u’=9, the amplitude of the second decay component is somewhat more important than for v’=3 and 4. Extrapolatron of the fast decay rate constant for u’ = 3-5 to zero Ne pressures grven mtercepts that are consistent wtth the N,(B) radiative hfetlmes [ 17 ] _ The decay of u’= 5 m A: &o IS shown u-r fig 3. Wrth argon the untial decay process IS so fast that It is virtually mdlstingurshable from the excrtatron pulse for intermedrate pressures and only the second slower component IS clearly evrdent. Thrs also IS the case for u’ = 3,4 and 9.
kg. 5. nuorescence levels after Nz(B,
curves in neon earner u+ 1) laser eucltatnon
gas
Nz(B.u) sharp spike
for some
The inltd
IS fluorescence from the laser dye that is observed even though the enuwon was wewed through the monochromntor.
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rescence from u = 4 (after u’= 5 excitation) and u = 6 (from u’=7 excrtation) is shown in fig. 5. The rrse time of the fluorescence and the tune for maxrrnum fluorescence mtensty is consistent with Nz(B, u’- 1) being associated with the slow N7(B, u’) decay component, i.e., the formation of N,(B, u’- 1) is too slow to be a dnect process from N2(B, u’). Analysts of the u’- 1 decay curves is complicated by the raprd coupling of N,(B,
15 January 1981
Acknowledgement I?us work was supported by the US Department of Energy and by the US Office of Naval Research.
We thank Professor S. Rosenwaks for communicating their laser-mduced fluorescence studies of N2(B) to us prior to publication.
u’- I) and the N2(W, u”) level. However. the exponenwmch probably tral decay of N,(B, u’- 1) fluorescence, corresponds to the formatlon process, seemed to be pressure dependent. FmaI resolution of the radiative and colhsional steps for the N2(W, u”) levels swats more complete analysrs and also have expenmental data.
References A L Lyutyb L-D. hfel’N!kova and N L. Sokolova. Soviet Phys. .J. 18 (1975) 46. r21 A.N. Wright-and d A. Wmkler. Active rutiogen (Academic Press. New York. 19681. [31 M F. Golde and i A. Inrush, Rept. Progr. Phys 36 (1973) 1285 r41 D.W. Setser and D H. Stedman, J. Chem. Phys. 52 (1970) [l]
3957.
4. Conclusions
151 L.A. Gundel, D.W. Setser. hf.A A. Clyne, J A. Covon and The present laser-induced
fluorescence
studies of
N,(B 3rIg, u’= 3-10) have demons!rated rapid, twostep, vrbratronal relaxation revolving a second electronrc state of N,. The second state IS identified as N,(W 3A,,) on the basis of the pattern of relaxatron of the individual N,(B, u’) levels. If thrs pattern of relaxatron also holds for the u’=O and 1 levels of N,(BsR ), the low levels also wrll be strongly coupled to N2(W 4 Au)_ These low N2(W) levels have very long raciiatrve lifetimes [18] and the N*(W) state may act as a reservoir state for the u =O and 1 levels of N2(B) in many expenmental situatrons. Indirect evidence suggests that the low levels of B and Ware collisronahy coupled to the A(3ZL) manifold of levels. The present work supports the general correlation suggested by Miller and Bondybey [8,91 that the ease of colhsronal crossing between states depends on both the FranckCondon factors and the energy defect. Further expenmental work is in progress.
308
[6]
W. NIP, J Chem. Phys. 64 (1976) 4390. T-D Nguyen, N. Sadeghl and J.C. Pebay-Peyroula,
Chem.
Phys. Letters 29 (1974) 242. [ 7 J G. Black, R L. Sharpless, T G Slanger and D C. Lorents, J. Chem. Phys. 62 (1975) 4266,4274 D-H. Katayama, T-A. hfiller and V E Bondybey, J. Chem Phys 72 (1980) 5469. [ 91 V-E. Bondybey and T-A. h¶iUer, J. Chem. Phys. 69 (1978) 3597. [lo] D-H. Katayama, T.A. Biller and V-E Bondebey, J. Chem Phys. 71 (1979) 1662. I1 11 N. Sadeghi and D-W- Setser, I. Chem Phys., to be published. [8]
[ 121 J.H Kolts and D-W. Setser, III: Reactive mtermetites 111 the gas phase, ed D.W. Setser (Academic Press, New York, 1979). [13] I. Nadler, D.W. Setser and S. Rosenwaks, Chem Phys. Letters 72 (1980) 536. [ 141 W-C. Clark and D W. Setser, J. Phys. Chem 84 (1980), to be published. [15] R.F. Heldner LH, D.G. Sutton and S.H Suchard, Chem Phys Letters 37 (1976) 243. [ 161 N. Sadeghi and D.W. Setser, Chem. Phys. Letters (1980), to be submitted for pubhcation. [17] M. Jeunehomme, J. Chem Phys. 45 (1966) i80.5. [ 181 A. Lofthus and P-H. Krupeme, J. Phys. Chem. Ref. Data 6 (1977) 113.