Dynamical symmetry breaking in the 4νCH rovibrational manifold of acetylene

31 January 1997

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 265 (1997) 244-252

Dynamical symmetry breaking in the 4UCHrovibrational manifold of acetylene M.A. Payne, A.P. Milce, M.J. Frost, B.J. Orr 1 School of Chemistry, Macquarie University, Sydney, N,S.W. 2109, Australia

Received 23 September 1996; in final form 26 November 1996

Abstract Time-resolved, fluorescence-detected infrared-ultraviolet double resonance spectroscopy of acetylene in the 12700 cm "4UcH' rovibrational manifold reveals unusual symmetry-breaking energy transfer, induced (at least in part) by collisions. This takes the form of odd-numbered changes of the rotational quantum number J, despite the fact that intramolecular transfer between the ortho and para nuclear-spin modifications of such a molecule is usually forbidden.

1. Introduction One of the most enduring molecular propensity rules for spectroscopic transitions and energy transfer is that which conserves the nuclear-spin symmetry labels of the initial and final rotational states [1,2]. In the absence of catalysts, interconversion of distinct nuclear-spin modifications is generally expected to be slow and inefficient, as is borne out in the classic case of ortho and para nuclear-spin isomers o f molecular hydrogen, H 2. However, it is known [ 3 - 7 ] that nuclear-spin isomerisation is more facile in some polyatomic molecules, owing to intramolecular perturbations and accidental degeneracies. In this paper, we consider the acetylene molecule, C2H 2. Its simple structure suggests [8] that its ort h o - p a r a nuclear-spin isomerisation is likely to be very slow and that collision-induced rotational relaxation [9] should therefore [1,2] show a strong propen-

I E-mail: [email protected]

sity for even-numbered changes of the rotational quantum number J, with o d d - A J transfer virtually forbidden. These expectations have already been challenged by our work [8,10] on rovibrational energy transfer in the highly perturbed 'Ucc +3UCH' region ( = 11600 cm - 1 ) of C2H 2, where o d d - A J transfer is observed and attributed to symmetry-breaking intramolecular perturbations. A more conclusive body of evidence of this type is now found in the 12700 cm ] ,4VCH, rovibrational manifold of C2H 2. The infrared-ultraviolet double resonance ( I R - U V DR) excitation scheme used is depicted in Fig. I. This shows the rotational energy levels (labelled J = 0 14) of the vibrational eigenstate derived from the (I 0 3 0 0 0 0 ) 0 E+ normal-mode basis state, with four CH stretching quanta excited - one of u I (or~) and three of u 3 (or+ ) - and superscripts denoting vibrational angular momenta [8]. The rotational levels are arranged in two stacks to distinguish those with even J , belonging to the ortho nuclear-spin modification (e, a symmetry, I = 1), from the odd-J para levels

0009-2614/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0009-261 4(96)0 1435-2

M.A. Payne et a l . / Chemical Physics Letters 265 (1997) 244-252

(e, s, I = 0) z. In the I R - U V DR technique [8], an infrared laser pulse excites a P- or R-branch transition (labelled 'IR PUMP' in Fig. 1) in the 12676 c m - 1 1)] + 3re 3 absorption band of C2H 2, preparing a non-equilibrium population in a particular J-state. Subsequent collision-induced energy transfer redistributes that population to other J-states, most readily through even-A J rotational energy transfer (RET) as represented by the vertical broken lines in Fig. 1. This redistribution of population into another rovibrational state J ' is probed by means of laser-induced fluorescence (LIF), with an appropriately delayed ultraviolet laser pulse exciting a transition (labelled ' U V LIF PROBE' in Fig. 1) in a suitably chosen vibronic absorption band of the A I A u - X ] ~ system of C2H 2. In our present experiments, we employ the 299 nm A - X •~0al~] rovibronic band (as 1J3"-'0 in a previous I R - U V DR study [13,14] of the 4~,CH manifold of C2H2); this implicates one quantum in each of the electronically excited vibrational modes v; (CCH trans-bend, 1048 c m - l , a~) and ~,; (antisymmetric CH stretch, 2862 cm - I , bu), followed by broadband (300-400 nm) detection of A.-X LIF. Details of the experimental method are as in our previous work [8]. This I R - U V DR approach [8-10] facilitates measurement of several different forms of spectroscopic feature, with their corresponding collision-induced growth and decay kinetics: (i) Parent peaks, where IR PUMP and UV LIF PROBE transitions are common to a single rovibrational level ( J' = J ). (ii) Regular R E T satellites, with J ' = J _+ 2 n (where n is an integer). (iii) Symmetry-breaking R E T peaks, with A J ( = J ' - - J ) odd, contrary to usual propensity rules. (iv) Rovibrational energy-transfer features, where J and J' belong to different vibrational states, in contrast to (ii) and (iii). The key result reported in this Letter is as portrayed in Fig. 1, where symmetry-breaking RET with

ConvenUonal spectroscopic symmetry labels that apply to the rovibrational levels of interest include: a / s nuclear-spin interchange symmetry, + and e / f parities, and g / u point-group inversion symmetry, See refs [8,11,12] for more information.

245

i*

J=12

,,,

- . . . .

)~.--. - -

-

tlq lib lit Ill

p

10

I

til II tl II

11 II .,

a

9

7 '~

UV

LIF

PROBE

6

5

,7 Et III

---, d'=]

even J

odd J

( e, a, ortho) (1 0 3 0 0 0 0 )

(e,

s, para)

0 ~u ROVIBRATIONAL

MANIFOLD

Fig. 1. I R - U V DR excitation scheme for studies of rovibrational energy transfer in the 12700 c m - J '4Vcn' manifold of C 2 H 2- The IR PUMP laser pulse excites the P(13) or R(11) transition in the v I +31.' 3 absorption band, preparing molecules in the J = 12 level. Collision-induced energy transfer redistributes that population to other states J ' , by e v e n - A J RET (vertical broken lines) and, as shown specifically, by A J = - 11 symmetry-breaking RET to J ' = 1 (monitored by the UV LIF P R O B E pulse). Other details are discussed in the text,

A J = - 1 1 appears to transfer population prepared in the (1 030000) 0 J : 12 level to the LIF-probed J ' = 1 level in the same vibrational state, but with an apparent change of nuclear-spin symmetry. This resuit, of type (iii), is a clearer case of collision-induced symmetry breaking than the odd-AJ rovibrational energy-transfer features of type (iv) that have been characterised [8,10] in the Ucc + 3uCH manifold of C2H2 at = 11600cm-1.

2. I R - U V

DR spectroscopic measurements

An extensive experimental search [15] has identified the 4VcH rovibrational manifold of C2H 2 at = 12700 c m - I as a region in which I R - U V DR provides a definitive view of symmetry-breaking collisional effects. As indicated above, the need for such a view of novel state-to-state energy transfer

246

M.A. Payne et a l . / Chemical Physics Letters 265 (1997) 244-252

processes had been established in our previous work [8,10] on the Ucc + 3 U c , manifold of C2H 2 at --- 11600 cm - l . In that earlier study, very few of the rovibrational states accessed have the combination of IR- and UV-brightness that is needed for reasonably strong I R - U V DR signals, so that the range of states amenable to investigation in the ~'cc + 3 ~'CH region is quite sparse [8]. This contrasts dramatically with the 4VCH region that is now examined. It was already known, from the preceding work of Crim and co-workers [13,14], that virtually all ( 1 0 3 0 o 0 o ) o J-levels are readily LIF-detectable in optical doubleresonance experiments, as is also the case in the 3~'CH [16,19--21], 2t'CH [16], ~'CH [16--18] and ~'cc [22,23] regions. Fully analysed details of our experiments on the 4~'CH region [15] will be published in due course. In the meantime, we report the most

I

I

I

I

298.7

298.8

298.9

299,0

~

I

I

2 9 9 . 1 299.2

I

I

I

299.3

299.4

299.5

_ I 299.08

I I 299.12 299.16 UV LIF PROBE WAVI::LI=blGTH(nm)

J='l I 299.20

Fig. 2. UV-scanned I R - U V DR spectra for the ( 1 0 3 0 0 0 0 ) 0 levels with (a) J = 1 and (b, c) J = 12. The spectra are recorded under effectively collision-free conditions with the IR PUMP tuned to the ~,~ + 3 v 3 P ( J + 1) transition and scanning the UV LIF PROBE wavelength through the A.-X 1°t335 o l J rovibronic band. Trace (c) is recorded with a seven-fold contraction of the wavelength scale used in traces (a) and (b). The 299.105 nm UV transition in trace (a) provides an unambiguous means of monitoring the (1 0 3 0 0 0 0 ) 0 J = 1 level.

remarkable set of results, which correspond specifically to the excitation scheme depicted in Fig. 1. Fig. 2 shows UV-scanned I R - U V DR spectra (as defined previously [8]) for the (1 0 3 0 0 0 0 ) 0 J-levels of prime interest, namely, (a) J = 1 and (b, c) J = 12. These are spectra of type-(i) parent features, each one recorded with the IR PUMP frequency centred on the P ( J + 1) peak in the 12676 cm -j u 1 + 3 u 3 rovibrational band of C2H 2 [24]. Each spectrum represents effectively collision-free conditions, with a C2H 2 pressure, Pc2n.,, of 50 mTorr and a short (10 ns) I R - U V delay t between UV LIF PROBE and IR P U M P pulses; this corresponds to a Lennard-Jones collision number zc2.2 = 0.008 for C 2 H 2 / C 2 H 2 self-collisions [8]. For J = 1, the outcome is a simple two-line spectrum (a), while the corresponding spectrum (b, c) of J = 12 comprises at least seven peaks, all of which are assignable to the .~-,X 10al~l - i ~,3-,0 rovibronic band [13,14]. It should be noted that there is an accidental coincidence between the longer-wavelength J = 1 peak at 299.145 nm and the central peak in the J = 12 spectrum; this tends to generate spurious results because both J = 1 (para) and J = 12 (ortho) levels are simultaneously monitored by the UV LIF PROBE radiation. However, the other J = 1 peak at 299.105 nm is found to be clear of peaks in the J = 1 2 spectrum (and, moreover, in UV-scanned I R - U V DR spectra recorded for any other value of J in the range 0-18). This 299.105 nm UV transition therefore provides an unambiguous means of monitoring the (1 0 3 0 0 0 o ) 0 J = 1 level. For the rest of the results reported, the UV LIF PROBE wavelength is held fixed on the 299.105 nm peak, thereby monitoring the (1 0 3 0 ° 0 ° ) ° J = 1 level exclusively (as depicted in Fig. 1). Corresponding IR-scanned I R - U V DR spectra (as defined previously [8]) are shown in Fig. 3, above a reference photoacoustic absorption spectrum (PAS) of the u~ + 3~, 3 band centred at 12675.68 cm -1 [24], shown inverted in trace (a). Trace (b) is recorded by scanning the IR P U M P frequency with PC2H2 = 200 mTorr, I R - U V delay t = 10 ns, and collision number Zc~,~ = 0 . 0 3 3 . These effectively collision-free conditions result in a simple two-line spectrum comprising only the type-(i) R(0) and P(2) peaks at 12677.98 c m - 1 and 12670.92 c m - ~, respectively, as expected when the J ' = J = 1 level is directly probed.

247

M.A. Payne et al./ Chemical Physics Letters 265 (1997) 244-252

P(2)

R(0)

R(11) R(17)

R'13" [ ) I

R(4) R(8) ,

I

I

I I

I

.....

I

P(4) I

I I

[ I

P(6) t

P(9) P(11)

P(13)

(C)

,

I

'°! (a)

I

12700

I

I

1268o 1266o IR P U M P F R E Q U E N C Y (cm "~)

I

12640

Fig. 3. Photoacoustic absorption spectrum (a) of the 12676 cm- t v~ + 3 v 3 band, shown inverted below two IR-scanned IR-UV DR spectra (b, c), recorded with the IR PUMP frequency scanned and the UV LIF PROBE fixed to monitor the (1 030000) 0 J = 1 level. Trace (b) is recorded under effectively collision-free conditions, with Pc:n: = 200 mTorr, IR-UV delay t = 10 ns, and Lennard-Jones collision number zc2,2 = 0.033. Trace (c) is recorded with a 50-fold increase in instrumental gain and a 20-fold increase in IR-UV delay, so that Zc2a 2 = 0.66; collisional interactions are seen to redistribute rotational population from various IR-pumped values of J to J' = 1, through a combination of even-A J RET and odd-A J symmetry-breaking RET.

T r a c e (c) o f Fig. 3 is r e c o r d e d with the same C 2 H 2 pressure, a 50-fold increase in instrumental gain, and a 20-fold increase in I R - U V delay. T h i s larger delay increases ZC2H2 to 0.66 and the c o n s e q u e n t opportunity for collisional interactions in the interval bet w e e n IR P U M P and U V L I F P R O B E pulses results in abundant redistribution o f rotational p o p u l a t i o n 3, f r o m various I R - p u m p e d values o f J to ( U V - p r o b e d ) J ' = 1. The m o s t p r o m i n e n t f o r m o f redistribution yields a series o f type-(ii) R E T satellites, corresponding to R ( J - 1) and P ( J + 1) transitions with A J = 2, 4, 6 . . . . ( c o n f i n e d to the para nuclear-spin m o d i fication). Far m o r e r e m a r k a b l e are two sets o f w e a k e r

3 Note that Fig. 3 shows spectral line intensities rather than actual rovibrational state populations, the latter being approximately proportional to the former by slowly varying, J-dependent HiSnI-London factors.

satellites centred on the R ( l l ) and P(13) peaks at 12699.89 c m - l and 12641.09 c m - I , respectively. E a c h o f these sets is based on transfer o f population f r o m J = 12 (ortho) to J ' = 1 (para), a c c o m p a n i e d by secondary R E T f r o m other e v e n - J levels, as anticipated in Fig. 1. T h i s o b s e r v a t i o n therefore corresponds w i t h little a m b i g u i t y to t y p e - ( i i i ) s y m m e t r y - b r e a k i n g R E T , in w h i c h o d d - A J e n e r g y transfer apparently effects o r t h o - p a r a nuclear-spin i n t e r c o n v e r s i o n in less than a full L e n n a r d - J o n e s collision. T h e r e h a v e been various reports [8,10,17,20,25,26] o f rotationally r e s o l v e d V - V transfer in C 2 H 2 , via type-(iv) I R - U V D R features. H o w e v e r , similar exp e r i m e n t s [15], h a v e failed to detect J - r e s o l v e d rovibrational energy-transfer between primary (1 0 3 0 0 0 0 ) 0 X + levels and the adjacent (1214000) 0 X + levels [24] to w h i c h they are a n h a r m o n i c a l l y coupled.

M . A . P a y n e et al. / C h e m i c a l P h y s i c s L e t t e r s 2 6 5 ( 1 9 9 7 ) 2 4 4 - 2 5 2

248

3. I R - U V D R kinetic m e a s u r e m e n t s

Further insight into the forms of rovibrational energy transfer that are evident in Fig. 3(c) can be gained by making state-to-state kinetic measurements. These are performed [8,10] by continuously scanning the I R - U V delay t (and hence the effective collision number z) while fixing the sample pressure and the IR P U M P and U V - L I F P R O B E frequencies. Depending on the combination of IR and UV wavelength settings, the growth and decay kinetics of each of the I R - U V DR features of types ( i ) - ( i v ) can be examined. It is already recognised [8] that a trivial explanation of symmetry-breaking rovibrational energy transfer could be in terms of intermolecular V - V transfer between states o f a- and s-symmetry, through the following collisional scheme (consistent with Fig. 1):

brational energy transfer in the ~CH [20] and 3I,CH [25] manifolds of C2H 2. Three pairs o f representative I R - U V DR kinetic curves are presented in Fig. 4. The uppermost trace in each section (a), (b), (c) of Fig. 4 is for neat C2H 2 with PC2H~ = 50 mTorr, while the lower trace is for a 1:10 mixture of C2H 2 in Ar with P c 2 , = 5 0 mTorr and PAr = 500 mTorr. In all cases, the UV L I F P R O B E laser pulse ( = 15 ns F W H M , generated by frequency-doubling an excimer-pumped dye laser

~oO.".. ... • .. .'.',~ •.

•

.

:,:,% °.""..'v.~ ..- .

.-.~.-.....~

GAIN+4 (a)

C 2 H 2 { ( 1 0 3 0 ° 0 ° ) °, e v e n J ; a } +C2H2{17", J,r"'s}

• j, ... "~.".'.-..~,'-..~.. j-.. . .. ~"~" "" "'~ "'" ~°"".'"~.',"""..'-...~',,-.--'--.'.'.~.v-..

~ C2H2{17', f ' ; a } + C2H2{(1 0 3 0 ° 0 ° ) ° , o d d J ' ; s}.

(1)

Here, the left-hand C 2H 2 species on each side o f the equation is the molecule that is initially state-selected and the right-hand C2H z molecule is the collision parmer that is probed, while 17" and 17' typically denote the ground vibrational state ( V = 0) or a low-energy bending state. Processes of this type are directly observed in low-energy vibrational levels of C2H 2 [27] and C z D 2 [28], and have been inferred [23] to explain apparent o d d - A J RET in the (0 1 0 0 ° 0 ° ) ° rovibrational manifold of C2H 2. However, such an intermolecular V - V mechanism seems improbable in the present case, since four CH stretching quanta vibrational quanta would need to be destroyed in one molecule and re-establisbed in the other, and more than 12000 cm -1 of rovibrational energy exchanged between the state-selected molecule and its collision partner. Experimental verification of this improbability has now been obtained by I R - U V DR kinetic measurements in which a foreign-gas molecule M, rather than C z H 2 itself, is the predominant collision partner. We note previous studies of the effect of C 2 H 2 / A r collisions on rovi-

(b)

/

°°,,"

•

.,

• ........... .. . . . .

""

I 0.0

. ~-....

...

(c)

.

|

"

,

I 0.5

,

I 1.0

,

I 1.5

IR-UV DELAY, t (Its) Fig. 4. I R - U V

DR kinetic curves recorded by scanning the

IR-UV delay t with the UV LIF PROBE monitoring the (1030000) 0 J' = 1 level ofC2H 2 and the IR PUMP tuned to (a) J = I (parent decay), (b) J = 3 (AJ = --2 RET), and (c) J = 12 (AJ = - 11 symmetry-breaking RET, as in Fig. 1). The uppermost trace in each section (a), (b), (c) is for neat C2H 2 with Pc2.2 = 50 mTorr, while the lower trace is for a 1:10 mixture of C2H 2 in Ar with PczH2= 50 mTorr and PAr = 500 mTorr. Vertical arrows in sections (b) and (c) show the values of t at which the total Lennard-Jones collision number ZtotaI equals 1.0.

M.A. Payne et a l . / Chemical Physics Letters 265 (1997) 244-252

[8]) is held fixed on the 299.105 nm peak in the ~ _ ~ •10al~l rovibronic band, thereby probing the iJ3J0 ( 1 0 3 0 o 0 0 ) o J ' = 1 level of C2H 2. The results in each section (a), (b), (c) of Fig. 4 are obtained by tuning the IR PUMP laser pulse (-~ 8 ns FWHM, generated by Raman-shifting a Nd:YAG-pumped dye laser [8]) to a chosen P- or R-branch feature in the l., l + 3v 3 band, so that the ( 1 0 3 0 0 0 0 ) 0 J-state is prepared. Each kinetic curve is recorded digitally by collecting the I R - U V DR signal as the I R - U V delay is scanned continuously [9]. Each data point is the average of at least 30 laser shots. The two traces (a) of Fig. 4 show the kinetics of the type-(i) I R - U V DR parent feature for J ' = J = 1; this is achieved by fixing the IR PUMP frequency on the 12670.92 cm -~ P(2) peak of the v I + 3~,3 band. The faster total decay rate of the lower trace of Fig. 4(a), relative to the upper trace, reflects the additional contribution of C 2 H 2 / A r collisions over a given time interval and its smaller amplitude is attributable to greater collisional quenching [29] of the fluorescence, of which separate measurements have been made [15]. The two kinetic curves in Fig. 4(b) correspond to type-(ii) A J = - 2 RET, with J ' = 1 probed and J = 3 pumped via the 12682.43 cm -1 R(2) transition of the ~,j + 3 v 3 band. The different growth and decay kinetics in the upper and lower traces reflect the relative efficiencies of RET in C 2 H 2 / C 2 H 2 and C 2H 2 / A r collisions. Finally, the two traces (c) of Fig. 4 are obtained by pumping J--- 12, using the ~'l + 3v3 R(I 1) transition at 12699.89 cm -1, and probing J ' = 1 as in the scheme of Fig. 1. Fig. 4(c) demonstrates that type-(iii) symmetry-breaking RET with A J = - 1 1 has a somewhat slower rise time than regular evenA J RET of type (ii), reflecting distinct kinetic schemes for even- and o d d - A J RET. Moreover, it is evident that the initial rate of growth of the lower trace of Fig. 4(c) (obtained with a 1:10 mixture of C2H 2 in Ar) is substantially faster than that of the upper trace (neat C2H 2, with the same value of PC2H2)" It is relevant in this context to define two forms of collision number: z M = k ~ P M t and Z t o t a I : (EMkLjPM)t, where PM is the partial pressure of species M, t is the I R - U V delay and k~j is the Lennard-Jones rate constant for C 2 H z / M collisions

249

at 300 K (11.3 p.s- t Torr- l and 16.4 p.s- J Torr- i for M = Ar or C 2 H 2 , respectively [25]). Vertical arrows on Fig. 4 show the values of t at which ZtotaI = 1.0, indicating that symmetry-breaking RET occurs on a collisional time-scale that is approximately gas-kinetic. Cursory inspection of Fig. 4(c) therefore suggests that addition of Ar as a buffer gas tends to enhance the rate of symmetry-breaking RET, contrary to what would be expected if the intermolecular mechanism of Eq. (1) were applicable. It is more realistic to propose an intramolecular symmetry-breaking RET mechanism, as follows: C2H2{(1 03 0 ° 0 ° ) ° , e v e n J ; a } + M

-, {C2H2(1030°0°)°,odd J';s) + M

(2)

This process can be promoted either by a foreigngas collider (e.g., M = Ar) or by self-collisions (M =

C2H2).

We have collected extensive I R - U V DR kinetic data of this type [15], covering a range of collisional conditions in mixtures of C 2 H 2 and Ar and incorporating corrections for collisional quenching [29] and ' b e a m flyout' effects [9,27]. A detailed analysis is in progress [15], using a rate-equation model designed to fit the kinetic results over a range of partial pressures to a set of mechanistic rate constants. In the meantime, we infer by inspection of Fig. 4 that the intermolecular mechanism of Eq. (1) is inappropriate on experimental grounds (confirming the theoretical improbability already noted [8]). We conclude on the same evidence that an intramolecular mechanism, represented by Eq. (2), must be adopted.

4. Proposed symmetry-breaking mechanisms The foregoing I R - U V DR results, both spectroscopic and kinetic, provide convincing evidence that symmetry-breaking energy transfer proceeds with high efficiency in the 4VcH rovibrational manifold of C2H 2. Moreover, it is possible to eliminate possible mechanisms - such as intermolecular V - V transfer, as in Eq. (1) - that would be of trivial significance. The proposed mechanism is an intramolecular process involving three steps (a), (b), (c) as depicted

250

M.A. Payne et al. / Chemical Physics Letters 265 (1997) 2 4 4 - 2 5 2

J=12

J=12

/

D I !

e-e

u-g a-s o~ho-para

IR PUMP

I !

e-

I

para - para

S--$

~

UVLIF

J=3

PROBE

J=2

e

(b) ~g-g

/ J=2

(c)

d=l

e-e

J=l

u-g

J=O

S--S

para - para

J=0

(', ozoOoO) ° I:~ 4 VCH (ungerade) (04)- C-H local mode

(2020000) 0

4V C H

Zg +

(gerade)

(04) + C-H local mode

Fig. 5. Mechanistic scheme representing I R - U V DR spectroscopy and kinetics of symmetry-breaking energy transfer in the 12700 c m - i 4UCH rovibrational manifold of C2H 2. The proposed mechanism is an intramolecular process involving three steps (a), (b), (c), the first and last of which entail transitions between the (04)local mode (ungerade) and the (04) + local mode (gerade) through dynamical symmetry breaking. Other details are discussed in the text.

in Fig. 5. This is an extension of Fig. 1; it involves two rovibrational manifolds at = 12700 cm - j , derived respectively from the (1030000) 0 E and (2020000) 0 E normal-mode basis states 4. These can also be written as 4Veil(U) and 4Veil(g) or equivalently represented in terms of C - H local modes

4 Our calculations, based on the method of ref. [30], indicate that the two vibrational eigenstates of interest at J = 0 are:

[13,31,32] as (04)- and (04) +, respectively. The 4UcH(U) and 4UcH(g) manifolds therefore differ simply in terms of the phases of the local oscillators. All of the rovibrational levels involved in this scheme are necessarily of e parity. The two levels involved in the final step (c), (04)- J = 1 and (04) ÷ J = 2, are known from spectroscopic data [24,32] to be split by - 0.47 c m - ~ and to display an anomalously large Stark effect. We postulate that, with their strong susceptibility to mixing in an electric field [32], they are also amenable to collision-induced dynamical symmetry breaking such as has been proposed recently [33] for transitions between ' + ' and ' - ' local-mode states. This step breaks the g / u symmetry but does not violate the a / s nuclear-spin interchange symmetry, with para ( I = 0) character conserved. A similar, but more speculative, mechanism has been considered (along with other possibilities implicating hyperfine coupling or vinylidene H2C=C:) in the context of oddA J energy transfer in the Vcc + 3VcH manifold [8]. The intermediate step (b) is a conventional case of collision-induced

RET

with AJ = --10 (or a corre-

sequence of even - A J steps), confined to the 4vCH(g) manifold. The initial step (a) in Fig. 5 is crucial to the mechanism. It entails the breaking of two symmetries that are usually conserved in energy transfer: not only g / u symmetry, as in step (c), but also a / s nuclear-spin interchange symmetry. The likelihood of such a postulate, and instances of such effects in congested electronic manifolds, have been discussed at length in ref. [8]. In this instance, we propose that there is a close coincidence between the J = 12 energy eigenstates of the (04)- and (04) + manifolds 4, thereby enhancing dynamical symmetry-breaking effects that involve nuclear hyperfine interactions and are expected to have small off-diagonal matrix elements. There is support for such a sponding

InucH ( u ) ) = 0.8541(1 03 0 ° 0 ° ) °) + 0.4871(3010°0°) °) + ... 14Vcrt( g ) ) = 0.7491(2020°0°) °) + 0 . 6 1 0 1 ( 0 0 4 0 ° 0 ° ) °) +0.181 X 1 ( 4 0 0 0 ° 0 ° ) ° ) + ... These are calculated to have term energies of 12673.1 e m - t and 12668.4 c m - J, respectively, compared to experimental estimates of 12675.68 cm -I [24] and 12671.55 cm - l [32]. There is one further 14UcH(U)> eigenstate at 13033 c m - l and two additional 14veil(g)> eigenstates at 12917 c m - i and 13230 c m - i.

4 With the 4.133 cm-~ difference between the J = 0 levels of (04)- and (04) + [32], this coincidence requires the rotational constant B for (04) + to be 0.0265 c m - i greater than the value of B for (04)- (1.15096 cm -~ [24]), all other parameters being equal. The coincidence could also be facilitated by centrifugal distortion and local perturbations.

M.A. Payne et aL / Chemical Physics Letters 265 (1997) 244-252

rovibrational symmetry-breaking mechanism from experiments [34] on hyperfine and 'superfine' interactions, in congested rovibrational manifolds of molecules such as SF6, and from theoretical studies of rotational energy surfaces [35] and dynamical interchange between local-mode states of opposite phase [33]. It is possible that step (a) is spontaneous and collision-free, such that the IR PUMP pulse prepares a superposition of (04)- and (04) + J = 12 states (with mixed g / u and a / s character). This situation cannot be distinguished at this stage from a fast, collision-induced process. To investigate the migration of population through the 4UCH(g) manifold, efforts have been made to use UV LIF PROBE transitions that terminate in gerade vibronic levels of the A state, but such IR-UV DR experiments are inconclusive so far. Likewise, it is conceivable that the detailed kinetic studies that are underway [15] might reveal an irregular pressure dependence, but such behaviour is not in evidence at this stage. Whatever the mechanistic details (Fig. 5), the striking feature of the experimental results (Figs. 2-4) is the net effect of relatively efficient energy transfer with A J = --11, induced (at least in part) by collisions, as represented by Fig. 1. The essence of the mechanism proposed is the fact [11] that the point-group inversion operator i is not a 'true symmetry' operator, because it fails to commute with hyperfine coupling terms in the molecular Hamiltonian; consequently, the g / u labels applied to molecular vibrational and electronic states define only near symmetry, which can be spoiled by perturbations involving nuclear spin. The unavoidable conclusion of this work is that there is an efficient rovibrational channel for a / s symmetry-breaking RET, and hence for interconversion between the ortho (I = 1) and para ( I = 0) nuclear-spin modifications of C2H 2. Moreover, the results seem to provide compelling evidence of previously predicted [33,35] dynamical symmetry breaking. Ongoing experiments [15] will address further verification of such mechanisms and modelling of their kinetics. It is fascinating to speculate as to how widespread (but hitherto largely undetected) dynamical g / u and a / s symmetry-breaking channels may be in congested rovibrational manifolds of polyatomic molecules.

251

Acknowledgements This project was financially supported by the Australian Research Council.

References [1] T. Oka, Adv. At. Mol. Phys. 9 (1973) 127. [2] J.T. Yardley, Introduction to molecular energy transfer (Academic Press, New York, 1980) pp. 279-287. [3] R.F. Curl, J.V.V. Kasper and K.S. Pitzer, J. Chem. Phys. 46 (1967) 3220. [4] J. Kern, H. Schwahn and B. Schramm, Chem. Phys. Lett. 154 (1989) 292. [5] P.L. Chapovsky, D. Papousek and J. Demaison, Chem. Phys. Lett. 209 (1993) 305, and references therein. [6] B. Nagels, M. Schuurman, L.J.F. Hermans and P.L. Chapovsky, Chem. Phys. Lett. 242 (1995)48. [7] B. Nagels, M. Schuurman, P.L. Chapovsky and L.J.F. Hermans, J. Chem. Phys. 103 (1995) 5161. [8] A.P. Milce and B.J. Orr, J. Chem. Phys. 104 (1996) 6423. [9] B.L. Chadwick, A.P. Milce and B.J. Orr, Can. J. Phys. 72 (1994) 939, and references therein. [10] A.P. Milce, H.-D. Barth and B.J. Orr, J. Chem. Phys. 100 (1994) 2398. [11] P.R. Bunker, Molecular symmetry and spectroscopy (Academic Press, New York, 1979). [12] H. Lefebvre-Brion and R.W. Field, Perturbations in the spectra of diatomic molecules (Academic Press, Orlando, FL, 1986). [13] A.L. Utz, J.D. Tobiason, E. Carrasquillo M., M.D. Fritz and F.F. Crim, J. Chem. Phys. 97 (1992) 389. [14] J.D. Tobiason, Ph.D. Thesis, University of Wisconsin-Madison, 1992. [15] M.A. Payne, A.P. Milce, M.J. Frost and B.J. Orr (unpublished results). [16] J.D. Tobiason, A.L. Utz and F.F. Crim, J. Chem. Phys. 97 (1992) 7437. [17] J.D. Tobiason, A.L. Utz and F.F. Crim, J. Chem. Phys. 101 (1994) 1108. [18] A.L. Utz, E.M. Carrasquillo, J.D. Tobiason and F.F. Crim, Chem. Phys. 190 (1995) 311. [19] M.J. Frost and I.W.M. Smith, Chem. Phys. Lett. 191 (1992) 574. [20] M.J. Frost, J. Chem. Phys. 98 (1993) 8572. [21] M.J. Frost and I.W.M. Smith, J. Phys. Chem. 99 (1995) 1094. [22] B.L. Chadwick and B.J. Orr, J. Chem. Phys. 97 (1992) 3007. [23] R. Dopheide, W. Cronrath and H. Zacharias, J. Chem. Phys. 101 (1994) 5804. [24] X. Zhan and L. Halonen, J. Mol. Spectrosc. 160 (1993) 464. [25] J.D. Tobiason, A.L. Utz and F.F. Crim, J. Chem. Phys. 101 (1994) 9642. [26] B.J. Orr, Chem. Phys. 190 (1995) 261. [27] B.L. Chadwick, A.P. Milce and B.J. Orr, Chem. Phys. 175 (1993) 113.

252

M.A. Payne et aL / Chemical Physics Letters 265 (1997) 244-252

[28] B.L. Chadwick and B.J. Orr, J. Chem. Phys. 95 (1991) 5476. [29] J.C. Stephenson, J.A. Blazy and D.S. King, Chem. Phys. 85 (1984) 31. [30] M. Abboutti Temsamani and M. Herman, J. Chem. Phys. 102 (1995) 6371. [31] M.S. Child and L. Halonen, Adv. Chem. Phys. 57 (1984) I. [32] J.A. Barnes. T.E. Gough and M. Stoer, Chem. Phys. Lett. 237 (1995) 437.

[33] G.M. Schmid, S.L. Coy, R.W. Field and R.J. Silbey, J. Chem. Phys. 101 (1994) 869. [34] J. Bord6 and C.J. Bord6, Chem. Phys. 71 (1982) 417; 84 (1984) 159. [35] W.G. Harter and C.W. Patterson, J. Chem. Phys. 80 (1984) 4241.