Collision-induced rotational energy transfer of CO (A1Π,v=3) with He, Ne and Ar: experiment via two-color 2+1+1 REMPI technique

Chemical Physics Letters 365 (2002) 244–250 www.elsevier.com/locate/cplett

Collision-induced rotational energy transfer of CO ðA P; v ¼ 3Þ with He, Ne and Ar: experiment via two-color 2 þ 1 þ 1 REMPI technique 1

Mengtao Sun *, Jie Liu, Weizhong Sun 1, Xiangling Chen 2, Bo Jiang, Guohe Sha State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China Received 22 April 2002; in final form 9 September 2002

Abstract Collision-induced rotational energy transfer of CO ðA1 P; v ¼ 3Þ with He, Ne and Ar is studied experimentally via two-color 2 + 1 + 1 REMPI technique. The propensity of  parity-conservation is observed. The cross-sections ! rðPþ ! Pþ Þ > rðP ! P Þ, and the abnormal phenomenon of r! DJ¼0 < rDJ¼1 for He and Ne are found. Relationship of r with DJ ; J and temperatures, and relationship of r with different partners are discussed. Ó 2002 Elsevier Science B.V. All rights reserved.

1. Introduction Rotational energy transfer is one of the simplest inelastic scattering processes in collision dynamics. Its profound influence in a diversity of gas phase phenomena has made it a hot topic in last couple of decades both in experimental and theoretical studies [1–3]. For the open-shell P-states diatomic molecules, the rotational energy transfer is more complicated than the closed-shell molecules because of K splitting [4–6]. Experimentally, a great

*

Corresponding author. Fax: +86-411-4675584. E-mail address: [email protected] (M. Sun). 1 Present address: AXT, Fremont, CA 94538, USA. 2 Present address: Department of Chemistry, The Ohio State University, USA.

deal of effort has gone into studying rotational energy transfer processes in 2 P and 3 P-state, such as NO ðX2 PÞ [7], CO ða3 PÞ [8]. Detailed experimental and theoretical investigations have been made on 1 P-state alkali dimers [9–11] focusing on an interesting phenomenon of quantum interference, which originates from the difference between the two K-related collision potential energy surfaces. Vidal [12] and coworkers have carried out two-step excitation experiments on rotational energy transfer study of CO ðA1 P; v ¼ 2Þ. The rotational energy transfer behavior of CO ðA1 P; v ¼ 1Þ state was studied experimentally by Sha et al. [13] via OODR-MPI technique. In this article, we report our detailed two-color 2 + 1 + 1 REMPI experimental results on rotational energy transfer of CO ðA1 P; v ¼ 3Þ, involving effects of different collision partners and

0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 2 ) 0 1 4 7 8 - 1

M. Sun et al. / Chemical Physics Letters 365 (2002) 244–250

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temperatures on the state-resolved integral crosssections, and  parity influence to the state-resolved rotational energy transfer cross-sections. Special attention is paid on a curious observation of the pure  parity change processes, and crosssections rðPþ ! Pþ Þ > rðP ! P Þ.

2. Experimental detail The experimental set-up was almost the same as previously detailed [14]. Two dye lasers (Lambda Physik FL2002, bandwidth 0:2 cm1 and pulse width 10 ns) were pumped by a XeCl excimer (Lambda Phsik EMG 200). The pump dye laser was equipped with a KDP frequency doubler to give an output wavelength 290 nm and pulse energy 1.0 mJ. The probe dye laser gives an output wavelength 560 nm and pulse energy several hundreds of lJ. The counter propagating and coaxial dye laser beams passing through two focusing lenses (f ¼ 15 cm) were focused into a static sample cell with a pair of built-in electrodes. The static cell is filled with

0.5 Torr CO + 10 Torr He or other partners, and cooled by a liquid nitrogen trap to eliminate the oil impurity influence. The ionization current drawn from the electrodes with 70 V bias voltages is amplified and averaged by a boxcar. The REMPI spectra are recorded with a PC data acquisition system. A typical spectrum of rotational energy transfer obtained for 0.5 Torr CO + 10 Torr He is shown in Fig. 1, where the two strong lines P (7) and R (7) originate from the parent state CO A1 Pðv ¼ 3; J ¼ 7; parityÞ and the weak lines come from collisional transfer to other rotational states. In the 2 + 1 + 1 two-color REMPI experiment of the B1 Rþ ðv ¼ 0Þ A1 Pðv ¼ 3; J ¼ 7; parityÞ X1 þ R ðv ¼ 0; J ¼ 9; parityÞ transition in the CO, the pump dye laser is set on the O(9) line of CO A1 Pðv ¼ 3; J ¼ 7; parityÞ X1 Rþ ðv ¼ 0; J ¼ 9; parityÞ transition while the probe laser is scanning the B1 Rþ ðv ¼ 0; þ parityÞ A1 Pðv ¼ 3; J ¼ 7; parityÞ, then which is ionized by absorbing one more pump laser photon. The rotational energy transfer cross-section can be obtained from the OODR-MPI spectrum on

Fig. 1. 2 + 1 + 1 two-color REMPI spectrum of CO B1 Rþ ðv ¼ 0Þ A1 Pðv ¼ 3Þ, where the pump line is O(9) of A1 P ðv ¼ 3; J ¼ 7; ParityÞ X1 Rþ ðv ¼ 0; J ¼ 9; ParityÞ. The gas mixture is 0.5 Torr CO + 10 Torr He and the temperature

253 K. The two strong lines P(7) and R(7) are from the parent state J ¼ 7, )parity. All the weak lines are the results of collision-induced rotational energy transfer.

the conditions of the single collision and saturation of detector laser [15] rffiffiffiffiffiffiffiffi 1 SðJ 0 Þ g pl 1 rjj0 ¼ ; ð1Þ ½Nm  SðJ Þ g0 8kT seff where ½Nm  is the number density of collision partner, l the reduced mass of M–CO, where M ¼ He, Ne and Ar, g and g0 the factors related to the statistic weight of lower state A and upper state B, and S ðJ 0 Þ/S(J) the ion signal ratio of daughter over parent species. An effective time seff for energy transfer can be expressed as: 1 1 1 ¼ þ ; seff sL sR

ð2Þ

where sL ( 10 ns) and sR ( 10 ns) [16] are the pulsewidth and radiative lifetime of CO (A1 P,v ¼ 3).

3. Results and discussion In our rotational energy transfer of CO ðA 1 P; v ¼ 3Þ;  parity rotational lines with J ¼ 2 up to 16 were pumped separately. Collision partners He, Ne and Ar were used individually to see the different partner effect. Experiments were

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performed under three temperatures, 77, 181 and 253 K to see temperature effect. In the following parts of this section we will summary the experimental results: the dependence of rotational energy transfer cross-sections on initial rotational quantum number J, rotational jump DJ ðDJ ¼ J 0  J Þ,  parity, temperatures and collision partners. 3.1. The propensity of  parity-conservation As in many other systems [13,17],  parity-conservation propensity is unambiguously observed for rotational energy transfer of CO A1 P with all above mentioned collision partner under the various temperatures. An example can be seen in Table 1 [the pump line is O(9) of CO A1 Pðv ¼ 3; J ¼ 7; parityÞ X1 Rþ ðv ¼ 0; J ¼ 9; parityÞ] and Fig. 2 [the pump line is P (8) of CO A1 P ðv ¼ 3; J ¼ 7; þparityÞ X1 Rþ ðv ¼ 0; J ¼ 8; þ parityÞ], where absolute cross-sections for CO–He, Ne and Ar under 253 K are displayed. The propensity is more notable for the smaller rotational jumps. The nature of the propensity of  parityconservation from the two different energy surfaces is physically sound. The scattering of an open shell diatomic molecule by a structureless partner is usually described by considering the symmetry of the electronic wavefunction. Because all three atoms lay in one plane, the total wavefunction can be either symmetric ðA0 Þ or antisymmetric ðA00 Þ with respect to reflection in this plane. In the symmetric case, the

open electronic orbital of CO is oriented in the atomic plane, and in the antisymmetric case perpendicular to this plane. One can construct a separate potential-energy surface (PES) for both symmetries, VA0 for the symmetric case and VA00 for the antisymmetric case. Alexander [6] has shown that for pure HundÕs case (a) diatomic, the sum P of these potentials V sum ðR; hÞ ¼ 12ðVA0 þ VA00 Þ ¼ l¼0 l Vl;0 ðRÞd0;0 ðhÞ describes the spin–orbit conserving transitions whereas the Pdifference potential l V dif ðR; hÞ ¼ 12ðVA0  VA00 Þ ¼ l¼2 Vl;2 ðRÞd2;0 ðhÞ describes the spin–orbit changing transitions. The quantum interference between V sum and V dif cause the propensity of parity-conservation [10,11], because the symmetric interaction potential VA0 for parity-conservation and the antisymmetric interaction potential VA00 for parity exchange are respectively, X X 1 1 V A0 ¼ Vl;0 ðRÞd0;0 ðhÞ þ Vl;2 ðRÞd2;0 ðhÞ ð3Þ l¼0

and VA00 ¼

X

l¼2

1 Vl;0 ðRÞd0;0 ðhÞ 

l¼0

X

1 Vl;2 ðRÞd2;0 ðhÞ

ð4Þ

l¼2

3.2. The cross-sections rðPþ ! Pþ Þ are bigger than those rðP ! P Þ It should be noted that the cross-sections rðPþ ! Pþ Þ are bigger than those rðP ! P Þ,

Table 1 2 ) for CO–He and CO–Ar of different initial J under 253 K Cross sections (A

CO–He

Parity

DJ

J¼2

J¼3

J¼7

J ¼ 11

J ¼ 16

!

)2 )1 1 2 )1 0 1

– 1.6 2.7 2.1

1.18 1.67 1.86 1.45 1.54 1.43 1.58

1.04 1.64 1.77 1.06 1.40 0.80 1.3

0.63 1.34 1.11 0.32 1.14 0.91 1.02

0.61 1.27 0.91 0.37 0.89 0.73 0.87

)2 )1 1 2 )1 0 1

– 5.6 6.0 4.8 4.1 6.5 5.0

2.23 4.82 6.17 3.36 4.04 4.81 3.38

2.13 3.92 3.37 1.91 2.26 2.71 1.96

2.01 3.25 2.84 1.04 1.99 2.34 1.80

– – – – – – –

!þ

CO–Ar

!

!þ

M. Sun et al. / Chemical Physics Letters 365 (2002) 244–250

if one compares the tables and figures. This phenomenon of unequally cross-sections for different parity was also found in molecular energy transfer [13,18–20] and photodissociation [21,22] processes. The physical origin in the K doublet propensity is a direct reflection of the fact that for the Pþ potential energy surface is more repulsive, whereas for the P potential energy surface is less repulsive than the Pþ potential energy surface [18]. The magnitude of the K doublet propensities is illustrated by calculated cross-sections for the CH ðX2 PÞ-He system [18] using the ab initio potential energy transfer surfaces calculated by the Argonne theoretical group, and these cross-sections are compared to those of the crossed molecular study of Liu and Macdonald [23]. A similar analysis was illustrated by inelastic collisions of OH ðX2 PÞ [18]. ! 3.3. The abnormal phenomenon of r! DJ ¼0 < rDJ ¼1 for He and Ne

We paid attention to an interesting phenomenon involving pure parity  change, which was noteworthy but has all along been ignored in previous rotational energy transfer of CO A1 P [12] and 1 P-state molecules [9,10]. As shown in Fig. 2, þ! þ! þ! rþ! DJ ¼0 < rDJ ¼1 for He and Ne but rDJ ¼0 > rDJ ¼1 !þ for Ar. As also shown in Table 1, rDJ ¼0 < r!þ DJ ¼1 !þ for He and Ne but r!þ DJ ¼0 > rDJ ¼1 for Ar. From Table 2, temperature does not alter this tendency.

247

We call this phenomenon an abnormality, since energy gap or momentum gap scaling law gives only the results of Ar. In rotational energy transfer study of NaLi B1 P, for all rare gases partners ! r! DJ ¼0 < rDJ ¼1 [9,10]. Physically, pure  change process is unique in that only the open-shell orbital is changed from the plane of nuclear rotation to out of the plane or reverse, but no energy is transferred if the very little energy difference between parity of the same J is neglected. 3.4. Relationship of r with DJ ; J and temperatures In Fig. 2, the cross-sections decrease exponentially on the whole with jDJ j increasing. These results reflect the exponential energy law [24]. From Table 1, for He and Ar as partners, r decrease also with increasing initial state J, the same results as in [25,26]. Ne (partner) follows the same trend. This is another reflection of energy gap law: the energy gap increases with J for the same DJ . Table 2 lists the temperature effect on r with He: the cross-sections decrease when the temperatures increase. This temperature effect on rotational energy transfer cross-sections is different from some previous observations on other systems [27,28]. Normally, when the bathing gas temperature rise, the availability of angular momentum from the relative motion of colliders increases,

Table 2 2 ) for CO–He under different temperatures Cross-sections (A Parity

DJ

T ¼ 253 K

T ¼ 181 K

T ¼ 77 K

!

)5 )4 )3 )2 )1 1 2 3

0.46 0.76 0.8 1.04 1.64 1.77 1.06 0.77

0.63 0.70 0.99 1.32 1.88 1.93 1.01 0.59

1.17 1.60 1.88 2.07 2.82 1.98 0.81 0.38

!þ

)2 )1 0 1 2 3

1.23 1.39 0.95 1.31 0.72 0.42

2.16 2.31 1.58 1.68 0.75 0.44

1.40 0.80 1.3

2hv1

The pump dye laser is set on the O(9) line of CO A1 Pðv ¼ 3; J ¼ 7; parityÞ

X 1 Rþ ðv ¼ 0; J ¼ 9; parityÞ transition.

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Fig. 2. (a) He collision-induced rotational energy transfer cross-section for initial CO ðA1 P; v ¼ 3; J ¼ 7; þparityÞ, T ¼ 253 K, PCO ¼ 0:56 Torr, PCOþHe ¼ 11:3 Torr. The pump line is P(8) of CO A1 Pðv ¼ 3; J ¼ 7; þparityÞX1 Rþ ðv ¼ 0; J ¼ 8; þparityÞ transition. (b). Ne collision-induced rotational energy transfer cross-section for initial CO ðA1 P; v ¼ 3; J ¼ 7; þparityÞ, T ¼ 253 K, PCO ¼ 0:56 Torr, PCOþNe ¼ 11:3 Torr. The pump line is P (8) of CO A1 Pðv ¼ 3; J ¼ 7; þparityÞ X1 Rþ ðv ¼ 0; J ¼ 8; þparityÞ transition. (c). Ar collision-induced rotational energy transfer cross-section for initial CO ðA1 P; v ¼ 3; J ¼ 7; þparityÞ, T ¼ 253 K, PCO ¼ 0:56 Torr, PCOþAr ¼ 11:3 Torr. The pump line is P (8) of CO A1 Pðv ¼ 3; J ¼ 7; þparityÞ X1 Rþ ðv ¼ 0; J ¼ 8; þparityÞ transition.

with an expectation of increase of r as is observed here. On the other hand, however, as temperature grows, the collision time interval is correspondingly shortened and the thermally averaged collision energy is increased. Both of these latter factors will reduce r. It is particularly the case for CO ðA1 PÞ-H2 =N2 collisions, where ÔExciplexÕ is readily formed [29]. Note that in our experiment, J is not so high and hence CO (J) does not rotate so rapidly that rotational averaging of the potential becomes important. This may be the reason for the

difference of the temperature effect between our experiment and previous [27,28]. We found that cross-sections for CO ðA1 PÞ is also asymmetric with respect to upward and downward DJ rotational quantum jumps for the same J, which is essentially caused by the quantum interference between K doublets [11], but this asymmetry does not oscillate with J, which was also found by Monchick [30] and Imajo [24]. Alexander considered that the thermal averaging in the static cell washes out the oscillation [31]. Cross-sections

M. Sun et al. / Chemical Physics Letters 365 (2002) 244–250

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Table 3 2 ) for different collision partners under 181 K Cross-sections (A

Fig. 3. Ar collision-induced rotational energy transfer crosssection for initial CO ðA1 P; v ¼ 3; J ¼ 11; parityÞ, T ¼ 253 K, PCO ¼ 1:08 Torr, PCOþAr ¼ 17 Torr. The pump line is O(13) of CO A1 Pðv ¼ 3; J ¼ 11; parityÞ X1 Rþ ðv ¼ 0; J ¼ 13; parityÞ transition.

for CO (A1 P, v ¼ 3, j ¼ 11)-Ar are shown in Fig. 3, where the profile of the cross-sections has a preference for DJ < 0, compared with that for DJ > 0, especially rDJ ¼1 > rDJ ¼1 . This is in contrast with the situation in c of Fig. 2, where rDJ ¼1 < rDJ ¼1 . This asymmetry is essentially caused by the detailed balance [32,33]. Under the experimental temperature (253 K), the rotational distribution maximizes at Jmax ¼ 7–8, so the system would reach this thermal distribution if enough collisions could occur. It is plausible that a single collision has a tendency to drive a single- level-populated parent molecules to the thermal distribution. Table 1 lists the crosssections for CO–He/Ar for different initial J state under 253 K. In comparison with the circumstances of J ¼ 7, 11 and 16, the rjDJ j asymmetry becomes reversed when J ¼ 2 and 3, which are smaller than Jmax . This Boltzman-maximum-distribution-preference propensity should be affected by temperature. This has been verified in our experiment by that the rjDJ j asymmetry changes accordingly as the temperature was lowered to 77 K, under which Jmax ¼ 3–4, and raised to 450 K, under which Jmax ¼ 9–10. 3.5. Relationship of r with different partners The same as in other investigations [34,35], the cross-sections of rotational energy transfer for CO

Parity

DJ

He

Ne

Ar

!

)2 )1 1 2

1.32 1.88 1.93 1.01

1.74 2.27 2.19 1.34

3.41 4.92 4.67 2.58

!þ

)2 )1 0 1 2

1.23 1.39 0.95 1.31 0.72

1.47 1.65 1.16 1.57 0.84

2.37 3.01 3.25 2.14 1.68

The pump dye laser is set on the O(9) line of CO 2hv1 A1 Pðv ¼ 3; J ¼ 7; parityÞ X1 Rþ ðv ¼ 0; J ¼ 9; parityÞ transition.

ðA1 PÞ show dependencies on the identity of the collider gas. This can be seen clearly from Table 3, which displays the cross-sections of DJ ¼ 0; 1 and 2 for the various buffer gases under 181 K. r increases monotonously in the series of He, Ne and Ar. One important factor is the increasing polarizibility of the He, Ne and Ar [29]. Larger polarizibility causes greater collision interaction, longer duration and hence larger cross-sections [2].

4. Summary In this Letter, we report our detailed experimental results on rotational energy transfer of CO ðA1 P; v ¼ 3Þ, involving propensity of parity-conservation, abnormal phenomenon of ! r! DJ ¼0 < rDJ ¼1 for He and Ne, relationship of r with DJ ; J and temperatures, and relationship of r with different partners.

Acknowledgements This work was supported by NNSFC (Grant No. 29973045) and NKBRSF.

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