Collisional removal of CD(A2Δ, B2Σ− and C2Σ+) by deuterated ketene

16 August 1996

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 258 (1996) 465-469

Collisional removal of CD(A 2A, B 2y_,- and C 2y_;+) by deuterated ketene Alejandro Alonso a, Milagros Ponz h, Margarita Martin

a

a lnstituto de Qufmica Ffsica "Rocasolano' ', C.S.LC., Serrano 119, 28006 Madrid, Spain b Facultad de Ciencias Experimentales y T~cnicas, Universidad San Pablo CEU, 28660 Boadilla del Monte, Spain

Received 31 May 1996; in final form 18 June 1996

Abstract

The collisional removal of CD(A2A, B 2 ~ - and C 2E+) by deuterated ketene has been studied. For several rotational levels of v = 0 of the A2A state, rate constants between 2.3 × 10-10 and 3.6 X 10- lo cm 3 molecule- + s - 1 are measured; the rate constants show a slightly increasing tendency towards levels with high rotational quantum number. The removal rate obtained for the unresolved rotational levels of AEA, v' = 2 is similar to that measured for the high rotational levels of o ' = 0 . The total removal rate constants measured for the B 2 £ - and C 2 ~ + states are 3.6× l0 -I° and 4 . 8 × l0 -10 cm 3 molecule-1 s - i respectively. For the B 2 ~ - state the rate constant shows no dependence on vibrational quantum number.

1. I n t r o d u c t i o n

The collisional properties of the lowest electronically excited states of CH have been studied to some extent because of their participation in the chemistry of a wide range o f systems [1-5]. Quenching studies of the electronically excited diatomic hydrides have provided information to test vibrational and rotational energy transfer models [6,7]. Other experimental [7] and theoretical [8] work has focused on the dependence o f the CH(A2A) removal cross section on rotational excitation and collision temperature. A systematic study o f the quenching of CH(B 2 £ - and C 2 £ + ) by several diatomic and polyatomic quenchers has concluded that reactive collisions are the dominant removal mechanism for the B 2 £ -

state [3], with a minor participation of near-resonant electronic-to-electronic energy transfer processes [3,9]; in contrast, electronic energy transfer seems to be the main quenching mechanism for the C 2 £ + state [3]. Much less attention has been given to the collisional removal processes of the electronically excited states of deuterated methylidyne; comparison with the undeuterated species could provide information to evaluate the role of the internal structure o f the collision partner [7]. As part of a wider study aimed to obtain information about the quenching properties of deuterated methylidyne, we have measured the removal rates o f C D ( A 2A, B 2 ~ - and C 2 ~ +) by the precursor molecule, ketene-d E. Owing to the way in which excited CD +s generated, by multiphoton

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A. Alonso et al. / Chemical Physics Letters 258 (1996) 465-469

dissociation of ketene, the collision process is likely to take place at high and non-thermal collision energies; this has to be taken into account when comparing data measured in this work with proper rate constants, obtained under well defined collision energy conditions.

persed light from the laser and from excited atomic carbon emission. Deuterated ketene was obtained from pyrolysis of deuterated acetic anhydric vapour (Cambridge Isotope Laboratories D6, 98%).

3. Results and discussion

2. E x p e r i m e n t a l

3.1. CD(A ~)

The excited deuterated methylidyne radical was formed in the ArF laser multiphoton dissociation of ketene-d 2. Ketene was photolyzed in a glass cell equipped with quartz windows; the laser beam was focused by a 2 m focal length lens, to a spot of -- 0.8 × 0.2 cm2; excited CD photofragment fluorescence, was observed at right angles with respect to the laser beam; the fluorescence was imaged onto the entrance slit of a 0.5 m Jarreil-Ash monochromator and the time resolved fluorescence viewed by a photomultiplier (Hamamatsu R928). Fast decay signals were recorded in a Tektronix 7934 storage oscilloscope (200 MHz, 1.8 ns risetime) and digitized by a CCD camera, Tektronix DCS01; averaged signals were transferred to a personal computer. Slower decays were recorded by a Tektronix 2430A digital oscilloscope (40 MHz, 12 ns risetime and 8 bits of vertical resolution); typically 128 decay traces were averaged before being transferred to a microcomputer. Pressures of ketene were measured by capacitive manometers MKS Baratron, type 227A and 221, with ranges up to 1 and 10 Torr respectively. In order to measure radiative lifetimes, experiments were carried out in the low collision regime of a free jet expansion. Ketene, kept in a slush bath (dry ice-acetone), was expanded into a vacuum chamber through a pulsed solenoid valve (General Valve Corporation, 0.5 mm diameter). About 10 mm below the nozzle, the molecular beam was perpendicularly crossed by the ArF laser; similar focusing conditions as in the cell experiments were used. Photofragment fluorescence produced in the interaction region was imaged by a lens onto the entrance slit of a 0.2 m Bausch and Lomb monochromator. Detection, recording and storage of the signals were as above. Appropriate cut off filters were use to reject dis-

Fluorescence decays were measured at different emission wavelengths of the Av = 0 band sequence of CD(A 2A ~ X 2H), with a spectral resolution better than 0.14 nm. The selected wavelengths are assigned mainly to R-branch emissions starting in levels N' = 11-17, 20 and 24 of v' = 0, overlapped in some extent by R-branch emission lines, with the same rotational labelling, belonging to the v ' = 1 v " = 1 band [10]; spectral simulation enables us to estimate in less than thirty percent the relative contribution to the total line intensity from rotational levels belonging to v'= 1. In all cases, traces could be well fitted to single exponentials. Measurements were also carried out at 432.1 nm, near the maximum of the Q-branch of the v' = 2 ~ v" = 2 band. Typical Stern-Volmer plots representing reciprocal lifetimes of decay traces versus ketene pressure are shown in Fig. 1. Rate constants and radiative lifetimes, obtained respectively from the slope and extrapolation to zero pressure of the Stern-Volmer plots, are given in Table 1.

4.5

.35 3.0

-

o

2.5

2.0

1.5 O.Oi

' 0.06

0.I 2

, 0.I 8

, 0.24

0.30

0.36

Pressure / Torr Fig. I. CD(A2A-~X 2II) emission. Reciprocal of the effective decay lifetimes versus pressure of ketene-d2: (a) for v' = 0; (b) for v'=2.

A. Alonso et a l . / Chemical Physics Letters" 258 (1996) 465-469 Table 1 Radiative lifetimes and removal rate constants for disappearance of CD (A2A, B 2X- and C 2 ~ + ) in the presence of ketene-d 2. Errors are two standard deviations Vibronic state

Rotational line

kQ(10-l0 cm 3 molecule- l s - i

7" (ns)

A2A, v ' = 0 , ( 1 )

A 2A, v' = 2 B 2X-, v' = 0

R(I1) R(12) R(13) R(14) R(15) R(I 6) R(17) R(20) R(24) Q-branch R(N' < 16)

2.3+_0.1 2.7+0.2 2.6 4-0.3 3.0 4- 0.2 2.9 4- 0.3 2.6 + 0.3 3.0 4- 0.2 3.1 4- 0.4 3.44-0.4 3.5 4- 0.4 3.5 +0.5

B 2X-, v' = 1

R(N')

3.65:0.3

C 2~+, v' = 0

Q-branch

4.84-0.8

567+ 17 631 + 3 4 568 4- 41 576 4- 34 574 4- 55 539 4- 37 560 4- 34 640 4- 94 7004- 114 525 4- 46 4144-64 530 a 4124-42 504 a 504-5 60 ~

a Measured in the free jet expansion of ketene-d 2.

Radiative lifetimes for v ' = 0, N ' < 20, are in agreement, within experimental errors, with values reported by Danielsson et al. [11]. For N ' = 20 and 24, we measure longer lifetimes than those given in Ref. [11 ]; in the latter, it is reported that rotational levels of CD(A 2A) have constant lifetime up t o N ' = 24, showing a small decrease at N'>~ 27 attributed to predissociation effects. A relatively high error is implicit in the zero pressure extrapolation method; alternatively, collisional effects might have been underestimated in Ref. [11]. However, we note that theoretical calculations for the C H ( A 2 A ~ X 2FI) transition predict an increase of radiative lifetime with increasing rotational level [12] that cannot be observed in CH due to predissociation effects. The radiative lifetime of v ' = 2 is of similar magnitude as the average values measured for the lowest N' in v ' = 0, at variance with C H ( A 2 A ) where a measurable shortening of the collision-free lifetime of v ' = 2, attributed to predissociation, has been reported [4,13]. The present result is in agreement with the conclusions of Ref. [11] stating that predissociation effects of the A 2A state of CD are weaker than in CH.

467

3.2. C D ( B 2,Z - , v = 0 a n d v = 1)

Fluorescence decays were measured at 386.5 nm near the head or the R t branch of the v' = 0 ~ v" = 0 band; within the spectral resolution of 0.64 nm selected, the recorded emission results from overlapping transitions assigned to rotational levels with 2 ~< N'~< 16 [10]. Measurements were also carried out at 368.0 nm with bandpass of 0.64 nm; in this spectral window, the observed fluorescence can be assigned to P, Q and R emission lines of the v' = 1 ---> v" = 0 band starting in rotational levels 0 ~< N' 15. For the whole range of ketene pressures investigated, fluorescence decays could be fitted to single exponentials. Stern-Volmer plots, representing the fluorescence decay rates for levels v' = 0 and v' = 1, are shown in Fig. 2. The total removal rate constants for each vibrational level obtained from the slope of the linear Stern-Volmer representations are shown in Table 1. Within experimental errors the removal rate constants show no dependence on vibrational level. Radiative lifetimes, measured in a free jet expansion of ketene, were carried out collecting broadband emission of the band sequence Av = 0; lifetimes measured in this way are --- 1.25 times longer than those obtained from zero pressure extrapolation of the Stern-Volmer plot. Work is in progress to try to improve the sensitivity of the beam experiments in order to carry out lifetime measurements over a narrower spectral region.

8.5

-r,n 6.5

t~ ~o

~'O 5.5 "" 4.5 3.5 2.5 1.5 0.0

' 0.1

' 0.2

' 0.3

' 0.4

0.5

Pressure / Torr Fig. 2. CD(B 2X- ~ X 2II)emission. Reciprocal of the effective decay lifetimes versus pressure of ketene-d2: triangles, o'= 0; circles, v' = 1. The values at zero pressure have been measured in the free jet expansion of ketene-d2.

468

A. Alonso et al. / Chemical Physics Letters 258 (1996) 465-469

3.3. CD( C 2~ +) Measurements were carried out at 314.4 nm, near the maximum of the Q-branch of the v' = 0 ~ v" = 0 band, with spectral resolution of 0.16 nm. Effective decay lifetimes were measured for pressures of ketene in the range from 60 mTorr to 1.2 Torr. Decay traces could be fitted to single exponentials; at times shorter than 20 ns after the maximum of the emission, decays are convoluted with the CD(C 2 X+) formation process occurring during the laser pulse; therefore traces were analyzed starting at times longer than --20 ns after the maximum of the emission. The data, plot in a Stern-Volmer representation, are shown in Fig. 3. The radiative lifetime measured in the jet expansion was in agreement, within experimental errors, with the zero pressure extrapolation of the SternVolmer plot. The values are compatible with the average lifetimes reported by Hesser et al. [14].

4. Final discussion and conclusions

The rate constants measured in the present work for removal of CD(A 2A and B 22~-) by deuterated ketene are, on the average, about a factor of two slower than those measured for the same states of the undeuterated species [4] and, for the A(2A) state, slightly increasing at the highest N'. The removal rate constant for v ' = 2 is of similar magnitude to

4.5

o0 ° ~0° 0 ~o / ~ ' /

4.0 ":m 3.5

o

3.0 'vt" 2.5

0o00 O~

o~ o 0

that for the highest rotational levels of v ' = 0, whereas quenching of the B(2E - ) state shows no dependence on vibrational quantum number. The efficiencies of the quenching processes are better examined by comparing the respective thermally averaged collision cross sections tr = k / ( v ) evaluated using the expression ( v ) = ( 8 k T / ~ r i z ) 1/2. The dependence introduced by the square root of the ratio between reduced masses of deuterated to undeuterated collision partners is only a factor of 1.03. The collisional temperature is more difficult to evaluate as has been pointed out in quenching studies of unrelaxed species produced in highly exoergic photodissociation processes [ 15]. The energy available to the products can be estimated as --200 kJ mol -~, assuming that methylidyne is formed following absorption of two ArF laser photons by ketene, leading to the products CH(A 2A), H and CO; however, it seems unlikely that the translational energy of the photofragment methylidyne changes by a factor of four from deuterated to undeuterated. Therefore we conclude that removal cross sections are more efficient for CH and ketene than for the deuterated collision partners. More work is needed, under better defined collision velocities, to know whether similar isotope dependence can be observed in collisions with other quenchers. The removal rate constant for CD(C 2 X + ) moderately increases with respect to that measured for the low-lying excited states; this is at variance with the behaviour shown by the undeuterated radical under quenching by several hydrocarbons [3]. In summary, the removal rate constants measured in this work indicate that the low electronically excited states of CD are efficiently removed by ketene, with rates slowly increasing with the energy of the excited state. Comparison with removal rates obtained for excited CH in collisions with hydrocarbons suggests that removal of CD occurs mainly through reactive processes.

2.0 1.5 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Acknowledgement

Pressure / Torr Fig. 3. CD(C 2~X+ ~ X 2 I I ) emission. Reciprocal of the effective decay lifetimes versus pressure of ketene-d 2. The point at zero pressure is the value measured in the free jet expansion of ketene-d 2 .

Financial support from Spanish DGICYT (PB930145 -CO2-01), Universidad S an Pablo CEU (project no. 13/95) and the Human Capital and "Mobility

A. Alonso et al. / Chemical Physics Letters 258 (1996) 465-469

P r o g r a m m e o f the E u r o p e a n C o m m u n i t y ( C H R X C T 9 4 - 0 4 8 5 ) is a c k n o w l e d g e d .

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[6] R.N. Dixon, D.P. Newton and H. Rieley, J. Chem. Soc. Faraday Trans. II 83 (1987) 675. [7] D.R. Crosley, J. Phys. Chem. 93 (1989) 6273. [8] A. Vegiri and S.C. Farantos, Chem. Phys. Letters 167 (1990) 278. [9] K.J, Rensberger, M.J. Dyer and R.A. Copeland, Appl. Opt. 27 (1988) 3679. [10] L. G~ro, Z. Physik. 117 (1941) 709. [11] M. Danielsson, P. Erman, A. Hishikawa, M. Larson, E. Rachlew-K~illne and G. SundstriSm, J. Chem. Phys. 98 (1993) 9405. [12] J. Luque and D.R. Crosley, J. Chem. Phys. 104 (1996) 2146. [13] T.A. Carlson, N. Duric, P. Erman and M. Larsson, J. Phys. B 11 (1978) 3667. [14] J.E. Hesser and B.L. Lutz, Astrophys. J. 159 (1970) 703. [15] R.D. Renner, F. Rohrer and F. Stuhl, J. Phys. Chem. 93 (1989) 7824.