Vibrational relaxations of NO by atomic oxygen

c

ii1

\il~

\I

1’111

I_ Introduction

Durkg the pat few years_ the rcl.Ltition of vLbr.~tionaliy excited molecules by free mdiczd atoms has been systematically investigated in sever~i IJboratorics f I] _ Howewr. the majority of these measLLrements have been made on mo1ecu1es, such as the hydrogen halidc2. in their L2’ ground etectronic st3tes. In such cases, the collision dynsmkr are li6eIy to be direct, with the transfer of an atom_ if it occurs, involving motion of the system over a potential brtrrier_ The esistence of such a reactive potentiat ~a11facilitate removai of the vibrationally escited species either by reaction or by non-reactive relaxation. Altema;ively. if the radical possesses eiectronic orbitai angu1.x momentum. its interaction with the molecule giires rise to more than one ekctronic potential and rel.wation can occur in electronicahy non-adiabatic collisions [121. When the vibrationahy excited species, as well as its collision partner, is a free radical, there e_xistsa third mechanism for rapid relaxation_ The two species _mayy, with high probability, combine to form a strongIy bound collision complex_ U&en this complex redissociates, in the absence of any appreciabIe barrier in the exit channel, the distribution over product states will probabfy be determined only by the conservation laws and by the relative volumes of phase space SOciated v&h different combinations of lima1states. In 218

SIC s 1 I I

I I KS

1 OLtubcr

1979

gcnernl. tmm.l~tioLud zuld rotational excitation of LILC producrs xvi11be preferred Lo vibrtltionzll excitation beizu~se of the higlier JeLLsity of such states. Conscquentiy. if the r.LLeof coLnpIex formation is independent of tile initial vibrational state of the moleculpr rttdiczu,and if energy is mndomised “immediateiy” the comples is formed, the rate constant for v~br.Ltional relaxation should be close to the limiting high pressure mte constant for combination of the two radicals [3,4]. ALLcscellent opportunity for stud> ing vibrational relfiation in the presence ofstrong attractive forces arises with the “stable free radicai” nitric oxide. The thermal combination reactions of NO with less inert radicals Iike II, 0, Cl, Br and 011 have been quite thoroughly studied [S] but the only previous measurements an vibrational relaxation of NO by any of these species were carried out at high tempemtures in z shock tube [6]_ The transfer of energy between NO@ = 1) and tloiz-radkal relaxants has been imestigated using a variety of techniques to excite NO [7-g] _ Our own method is similar to that employed by Green and Hancock [S]. They excited NO by vibrationalvibrational (V-V) energy transfer from HF(u = 1) vhieh was itself excited by irradiation with a pulsed HF chcmieal laser_ We decided to use HCI rather than HF_ This avoids any problems associated with handling HF, as well as kinetic compliations which might arise if V-V energy transfer between HF(u = 1) and NO promotes NO to u = 2 in a near-resonant eschange

process [S]. Although the rate wnst.mt lICl(u=

1)~xo(il=o~~I1cl(v=o)iso(u=

for 1)

(1,

is. st k, = 9.6 x IO -1-l crd illolecule-~ s-1 [IO]. about half that for the corresponding process with [IF. tltts c.tuses no dtfficulty. and the vibration.&-translatlonaf reL.-l.lxttion of KO(u = 1) by lIC1 is certainly no faster than its rela.ttion by I IF.

2. Experimental The .tpp.mttus used in the present experiments was very simi1.x to that we have used to me~surc the reLation cf I IBr(u = 1)_ DBr(u = 1) .md I ICl(u = 1) by Br atoms [ 1 I 1. A single pulse of radiation from .I discharge-imtiated LICI chemical Ixcr was directed along ;1 I15 mm internal diameter flowtube_ Infixed tluoresfence front iiO(u = 1) was observed at right nn$-s to the Lser beam through a C.IF, window_ The liquid-S, cooled InSb photoconductive detector was preceded by ;1 filter x\hich absorbs wavelengths < 4.3 ,um_ This removed the fluorescence from HCI and any scattered laser light, s well as the continuous emission ziso&ted with the 0 + X0 recombination. The signal from the InSb detector was amplified, sent through 4 tr.msient recorder_ and stored on paper tape_ In the absence of osygen atoms, the sign&to-noise ratio at the peak of the fluorescence signal was roughly 20 : I_ This decreased to about -l : 1 at the highest concentrations of atomic o_xygen. To compensate for this, up to 4 experiments were arried out in close succession on the sane misture, and the results were added together before tlml>sis by computer_ To generate oxygen atoms. ;t mixture of O-3--2% of O2 in Ar was passed through ;L 20 W microwwe discharge upstream of the min flo\\tube. The flowrate of 0 atoms, and hence their initial concentration [Olo. ws determined by titration with NO7 _ The ful1 titration procedure [I I] was carried out, ali&ving [Olo to be determined within approsimately t 3% Having established [010, the flow of NO, was turned off and a mixture of 45% HCI and 55% CO added to a combined concentration of approximately 5 X 10ls molecule cm -3, through an inlet 6 cm upstream of the infrared detector_ This distance allowed complete mi_\ing but minimised the removal of 0 atoms by thermal reaction_ Nevertheless, a reduction of IO-15% in

the concentration of 0 Jtom_s had to be allowed for_ becanse of the reactions: 0 + SO + Xl - NO, f A1and T NO [S] _The reaction, 0 t HCI otN02 -01 OH t CI. is too slow [5] to reduce the concentration of 0 atoms signifkmtly. The experiments \\ere all perfornted dt roont teniperature, (796 = 3) I(_ and ;It a total pressure of 3.5 Torr_ 1Ki. X0, and Ar were purified by procedures Jescrlbed previously [ 11_121_ NO w.ts taken from a cq linder (,\lsthcson, researclt grade). passed slowfy through two traps cont.tining grade 5X niolecuIar sieve at 195 K rend then frozen at 77 K and degassed.

3. Results The analysis of infrared fluorescence experiments where two different species are coupled by V-V energy eschange is often compliated and it is sometimes impossible to give 311 unequivocal interpretation of the results [ 13]_ In the present system. however, III;LIters xre greatly simplified because the trmsfer of enera from tiCi(u = 1) to NO is h@ly exothermic (LX= -1010 cm-t). The rare constrlnt for the reverse process, i.e. (-1). is only 7.1 X IO-l6 cm3 molecule-* s-1 so that, even in the absence of atomic oxygen. the rate of (-- 1) is negligible compared with that of other processes removmg NO(u = l)_ As a result the rate equation for the concentration of NO(u = 1) is simpIyr d[NO(u = I)]/dr = k, [NO] [HCl(u = I)] - litst [NO(u = I)] -

0)

The pseudo-l&t-order rate constant k,,, takes account of all processes which remove NO(u = I) including XO(u=

l)tO+-NO(u=O)+

o_

0)

Furthermore, because the equilibrium (1, -1) lies far oker to the right (K = k1/k_1 = 1X), the concentration of HCI(u = I) decays essentially 3s a single exponentitll. The integration of eq_ (I) is therefore simple and yields: [XO(u = I)] = X {exp(-k&t)

kl [NO] [HCl(u = I)],=,, (&t

- k;St)

- exp(-ll-t,,r))

where li;,t is the first-order

,

rate constant

@J for the over-

In prJsticc_ the digitBed e~I~crimcr:t31 bigmds kkerr titted to rhc functiorr.d form of ecI_ (II) wing a cwnputs: program_ This procedure yields two first-order r.ne con>t;lnts hut does not itself deterrninc \\hwh is , kt,I JIl3 \rhIctl is x-t,. In rhperutkw~s \\ltIl 3tomw o_\?_;cnpresent. there w3s oo difficulty wer this assignment. TIiC sftkct ofrtdding quite sm.dl concentrttions 0t 0 .itoms \\2sro reduce the intcn5ity of the NO tluorcsccnce 3nd to accelerate sllarply the mte 3t which it rose to its m3Gmum \rtlue_The raultant incre3ssein the wIcuLncd value of the r3te constant determined by the rLr of the NO tluores~znce w.ts mucl~ too grnt to hr assoc.-irttcdwith removal of I ICl(u = I ) by 0 3toms. for which the rate const3nt 4t room tempeziture is (9 2 I) X 1O-*5 cm? mokcula-* s-t [I]_ The insensitivity of the L&Y-U-L* rate of the x0 fluorescence w.rs ho\\ever consis:enE with this vdue_ In 3ddition, the correct ev3luation of X-ist ~3s checked by making 3 few observations on the simpie esponential de~+s of the fluorescence from I ICI(u = I )_ Analysis of the NO fluorescence in the absence of zltomic oxygen g;lve first-order rate const3nts which were much closer together: typicidly, ==600 s-1 and -500 s-t _ On the bttsis of published relaxation data for HCi(u = I ) f7,10] 3nd NO(u = I) [S,9], our own measurements on the rektxntion of NO(u = I) by O2 [I I ] ~3nd obsetwtions on the HCI(u = I) fluorescence, we took the larger of these mte const.mts to correspond to $st_ In any use, an incorrect assignment at this point hrts negligible effect on the vzllue we derive for k2_ A microwsve discharge through O2 produces O&+) in similar concentmtions to atomic oxygen. To e*nsurethat these rnektstabie molecules do not reItLv X0(21 = 1) at a significzmt rate, 3 few experiments were performed with the discharge on but with just suftkient NO2 3dded to extinguish the air afterglow_ This procedure removes 0 3tom.s but not O,(I+) [IS]_ The rate of deactivation in these expe>ments wzs the sme 3s in those with the microwave discharge switched off_ V3htes of kl, at different concentrations of atomic

oxygen were determined in sever31 series of elperi-

ments performed on different days_ The rrsuits are >hom n in fig_ ! _ k tsr varies linearly with [0] And the gradient of this line yieids: k2 = (6-5 * O-7) X IO-xt cm3 molecule-*

s- *

.ts the rate const.mt for c eactiwtion of NO(u = 1j by 0 atoms at (296 r 3) Ii_ The error quoted corresponds to cwu standard deviations.

4. Discwsion The Isrge v.tIue which we find for k,, corresponding to energy transfer in 3bout 3 quarter ot‘the g3s kinetic collisions between NO(u = 1) zmd 0 3toms, is most readily explained in terms of 3 mechanism involving the formation of bound (NO?)? complexes. When this is the case, the rate constant for formation of products in 3 st3te ut- czln be written [3,4] :

where kc is the second-order rate constant for complex formation and kc_,f the first-order rate constitnt for dissociation of the complex to yield NO in the vibr3tion3l state uf_ Where creation of the complex proceeds by way of it loosely bound complex, kc is un-

Iikely to depend strongIy

on Ui, the initial vibrational

V&m~e

of X0. and is also 2ssentiJly iLldependent of temperature [ 17.18 ] _ 1luwewr, LS the tempcr~tt~re. z111d hence the mem energy of the complexes, is incre.wd. d hrg2r frxtion of the co~iiplews wll redissociatc without NO liavinz been rela2d. so tli.Lt uf=ui_ Sewr.d methods an be used to estim.Lte relative values of-k‘_“* _Quack and Tree [q] sugg2st that. for the c.k.2 of decomposition of 1 Lriatomic complex LhroLLglL;1loosely bound twnsition SL.ILC?, these r.LLe constants art proportionA to (f?- i:;,), where f?= Ewi f < RT. i-2.. the IIEIII energy of the cornpIes .Lbow that required to dissoci.Ltr, in&ml rotationA energy beirlg neglected. Appiying this formula yields: Q = k,__,L= 1!k,__,p = 0-l-l .Lt300 K and 0.6 at 2700 K_ Xlrern~tivzly. one can adopt a semi-anpirical apIxoac11 [lOI basrd on RRK theory. Then Q is @en by [~~IRTT/(E~~,~- Eutzo t ~IIRT)]*-~ \+here UIRT is the mean energy contributed to the comples by reIaLi\e tr.mslaLion and rotation of the reagents_ The value ofs- 1 is estimated .~s==2 from considerations of the temperature dependence of the recombination nxction at low pressure. and then Q ~0.05 at 300 K aLId 0.51 at 2700 K_ Finally, an estimate of Q cm be made using equations tabulated by Levine and h1.m.z[?I] for prior detailed rate constants, based on the supposition that these depend only on the relative volumes of phase space avZlabl2 to products in different combinations of final stata. This approach leads to estimates of e of O-OS at 300 K and 0.64 ;tt 1700 K_ Our experimental value of kl cm be compared with the value obtained at =2700 K by Glanzer and Tree st3te

Tab!r

1 October 1979

CItl:\lICAL PIIYSICS LL-t-l-IIRS

66. number 1

[6] and given in table I_ Remembering that at 2700 K there is .Lnappreciable probability that NO 2merg2s from NO t 0 coilisions with uf > ui as well as with uf = u,, it is clear that the nrgative temperature dependeLlc2 iLtdicat2d by this comparison is entirely consistent 11ith d reksation Lnechtlnism involving collision complews_ The r.Lte constant for rekxxztion at room temperature can also b2 compar2d with rhose for 0 f X0 recombirlation in the limit of high pressure (kFec) and for isotope exchnnge berween 180 + N160(ke,ch). “C-L should correspond to kc and shoufd be close to k, . if complex formation and dissociation is the main roLlLe for relaxation of NO(u = 1) and if k, is independent of uI_ Troe [ 161 has measursd the rate of 0 + NO r2combinJtion st pressures up to 100 atm. Nevertheless, ;I long extmpolation is r2quir2d to deduce kTe, .Lnd agr22ment \\ithin a fxtor of about 2 with our value of k-, must be deenled sstisfkctory_ It seems r2ason~ble LOsuppose tlmt the isotope exchange reaction proceeds vi.L t80N160 compleses. Since. these can decompose “back” to IgO -t Nt60 as well as “forward” to N1”O f 160, k, should be appro_ximately twice ke\ch- This would Iead to an estimate of k, just o\er half our value of kz_ Of course_ it is possible that k, increaxs signifiwntly with ui, although this is unlikely as rhe transiti state leading to complex formation is apparently loosely bound, or that o*&er mechanisms contrIbute to the relaxation of NO(u = 1) by 0 atoms_ However, in view of the experimental uncertainties, such ;I conclusion would be premature. Table 1 also includes theoretical estimates of kFec and k, made by Qwck and Troe [1?,18,3,4]_ Both

1

CompxLon of rate comtdnts Temperature (K)

Reaction

__._---__

__0

-I-

180

NO(+Jl)

+ h’160-

NO<“=

-

--

--

hO:(+M)

N=O

+

l)-cO-NO(u=O)+O

=

2.6

300

l.Sa)

300

2-l b)

7-700 300 300 396 i 3

cm3

molecule-’ s-’ )

300

310

I60

k(IO-1x

1.8

3.7 -c I.7 1.7 =) Mb) 6.5 T.0.7

a) Calculated using the smtistical adiabatic channel model. b, Calculated using transition sfxe theory, the maximum free energy criteria. and an interpolation functions along the reaction path.

Ret [16-IS1 1171 iIf31 (191 [61 r;f L tiis work

procedure

to estimate partition

221

References

Taking a, = “FiC, Quxk and Tree estimate xdues ofk, at 300 K which are 3-4 times snlsllcr th.m our observed vrtIue_ It appesrs. however, thtlt this &scrcpancy cJn bs explained within the framert,ork of QuxL zmd Tree’s model. if one takes xcotmt of rextion via the essited etecrronic states of KOz_ Including ~111the spin-orbit components of boih species, inttxtction between KO(Xzll) 2nd 0(3P) @es rise to no iess thn twelve electronic terms. six doublet and SLYquartet states_ Extensive multi-configuration seIf-consistent field cafcuIations [221 show that, ;IS well as the %?A1 ground state, st least three excited doublet states Jre stabIe with respect to dissociation to NO(X ?I!) r 0(3P)_ As tong as the adiabatic potential for each of these states increases monotoniatly along the reaction path to the dissociation limit, there \\iII be significant cxmtributions to rhe overaIl dissociation and recombination rates from these additional electroniaiIy adiitbatic channels for reactionFoiIo\cing Quack and TroeS method [4,lS] with their recommended value of y = O-75 a-t, we have estimated these extra contributions to the reaction rate, using data for the excited states of NO, calculated by GiIIespie et aI_ [El. At T= 300 cm3 moIecule-I s-t K, we find k;,, = 5.8 X IO-” S-I _ and hence 11-?= 50 X 1O-i* cm3 molectde-*

C hem. %c_ 53 (197117. 11. QU;ILL and J_ 1 rw. Rer. lfunscn_rer. l’hy~d.. Chem_ 79 (1975) 170. \I. Qu;lclr .md J. Trot. Ber. Bunscngn. l’h)ak Chcm. 81 (1977) 160. 12.1 _ Ikntpxm Jr. .md D_ &r\in. cdr.. Re.vztion Rare .md l’h~~~whcmiC;Il Data f-or _Atmosphcric < hcmi>tr> 1977. XBS Sfwcixf Publication 513 tlS?S). I;. Cl~nrer ;~nd J. rroe. J. Chem. i’hrs. 63 (1975) -X351_ I‘. Weitr and <;_ 1 1) nn. Ann. Ret_ l’hr s. (?ial. IS f 197-f) 175. W-IL Crtxn .md J.K_ lkmcocf., IL 1-L‘ I_ r.kuntum f:tccIron. QC-9 11973, SO_ J.C_ Stcphmson. J. Chcm. I’h>s. 59 I 1973) 1513: 60 l19W) -f189. I’_I;_ Zittel And C-B. ~loore. J_ Chcm_ I%? 5. 28 f 1973) ,971. K.P_ t ernando xxi I_W_Y. Smith_ J. Chcm. SOL-.1 .tr.tdJ> II 75 11979) 106-Z_ R.D.fL Brown, <;.I’_ G&s and I_W_\l. Smith. J_ Chcm. Sot_ I-arad~~ 11 71 (1975) 1963. [ I3 I J-C_ Stephenson and C-II_ Moore. J_ Chcm_ Ph\ s_ 56 (1972) 11%. [I-Z J R-P. I‘ern;mdo and I_\~X. Smith. 10 be pubIishrd_ [ 15 I J-X. Davidson and CA. Osryzfo. in: Chemilummescmrr and bioluminaxnce. eds_ NJ_ Cormier. D.\l_ lkrcuics end J- Lee Wlenum Press. X\‘ca York, 1973) p- i 1 l[ 16 1 J. Trou, Urr. Bunsengc~. Pf~ysik. Chem. 73 (1969) 906. 1171 .\I. Quack 3nd J. Tree. Ber. Bunsenges. Physik. Chem. 79 ( 1975) 170. [ 18 J 31. Quack and J. Trot. Ber. Bunscnga. Phyxk. Chcm 81 11977) 3’9. [ 19 J J-T_ Herror! and I-S. LIein. J. Chrm. Phys. 10 f 1964) 273x_ [XII D-K Jaffer and LWX. Smith, Faraday Discussions Chsm. Sot. 67 (1979). to be published. [X 1 R-D. Levine and J_ Man.z, J. Chem_ Phys_ 63 (1975) 4280_ i221 G-D. Gdlespie. A-U. Khan. AK. Wahl. R.P. Hosten) and 21- Prauss. J- Chem- Phys. 63 I 1975) 34X_