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Development of a highly intense state-selected CH radical beam source for the study on the collision energy dependent reaction dynamics
Chemical Physics Letters 421 (2006) 124–128 www.elsevier.com/locate/cplett
Development of a highly intense state-selected CH radical beam source for the study on the collision energy dependent reaction dynamics Y. Nagamachi, H. Ohoyama *, K. Yamakawa, T. Kasai Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama-chyo-1-1, Toyonaka, Osaka 560-0043, Japan Received 16 December 2005; in final form 10 January 2006 Available online 8 February 2006
Abstract A velocity-variable highly intense pulsed supersonic CH (X2P) radical beam source was newly developed for the study on the collision energy dependent reaction dynamics of the state-selected CH radical. CH radicals were state-selected by an electrostatic hexapole field. The focusing curves and the CH flux intensities for the jJ, Fi, Mæ states were measured by a saturated laser-induced fluorescence spectroscopy. As a demonstration, the rotational state dependence of the reaction cross section of CH + O2 ! OH(A) + CO reaction was determined at the collision energy of 0.06 eV. 2006 Elsevier B.V. All rights reserved.
1. Introduction
[12]. In the previous CH source, the following reaction schemes were adopted to produce CH radicals.
Methylidyne radical (CH) is one of the most important intermediates in many fields including combustion, atmospheric, interstellar. From this reason, a number of studies on its reaction rates have been carried out [1–6]. In contrast with its importance, however, the studies on its reaction dynamics have been extremely limited [7]. Surprisingly, even for most of the fundamental reaction systems, the reaction products have not been identified. This is because the radical source always becomes mixture of reactive species in the process of CH radical generation [8–10]. From theoretical and practical importance of the CH reaction, it is worthwhile to develop a high-intensity, high-purity state-selected CH radical beam source. Recently, we succeeded in developing a highly intense and pure CH radical beam source using an electrostatic hexapole field [11], and applied it to the elementary reaction of CH + NO, O2 at the collision energy of 0.14 eV
He ! He ð23 SÞ
*
Corresponding author. Fax: +81 6 6850 5403. E-mail address: [email protected] (H. Ohoyama).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.01.025
3
ð1Þ 1
þ
1
He ð2 SÞ þ COðX R Þ ! Cð DÞ þ O þ He 1
2
Cð DÞ þ H2 ! CHðX P; v ¼ 0Þ
ð2Þ ð3Þ
Although this CH source can produce a high-intensity and high-purity state-selected CH radical beam, the source has a disadvantage that the velocity of CH beam is not variable by changing seeding gas, because of the extremely poor efficiency of CH formation by using other rare gas except for He. For the better understanding of the reaction dynamics, it is very important to control the collision energy. In the present study, we developed a velocity-variable highly intense CH radical beam source. The CH beam was characterized by a saturated laser-induced fluorescence spectroscopy (SLIF) in the A2D X2P transition [13]. The maximum flux intensities of the CH radical in each rotational jJ, Fi, Mæ states, were determined by using SLIF, where J is the total angular momentum. M is the projection of J on the quantization axis and Fi designates the fine state [14]. As a demonstration of the usefulness of the velocity-variable CH beam source, CH (X2P) + O2 reaction was studied at the collision energy of 0.06 eV.
Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
2. Experimental The experimental setup is almost same with the previous one except for the CH beam source [11,12]. The inefficient CH formation in the previous CH source by using other rare gas except for He should be caused by the extremely inefficient C(1D) formation in the reaction (2). The efficiency of C(1D) formation using Ne and Ar was estimated to be at least 2–3 order lower than that using He (i.e. out of the detection limit by SLIF). In order to overcome this disadvantage, a new approach using the reactions described below was carried out. Rg ! Rg
ð10 Þ
Rg þ CðsolidÞ ! Cð1 DÞ þ Rg
ð20 Þ
Cð1 DÞ þ H2 ! CHðX2 P; v ¼ 0Þ
ð3Þ
In the newly developed CH source, C(1D) is generated by the collision of solid carbon with the highly excited states of rare gas, Rg**. Based on the heat of sublimation of solid carbon (7.41 eV) and the excitation energy of C (1D) (1.26 eV), the threshold for the reaction (2 0 ) is estimated to be 8.67 eV. Therefore, the formation of C(1D) by Ne** and Ar** is energetically possible. Fig. 1 shows a schematic view of the CH beam source we have newly developed. The beam source consists of two parts. First part is a discharge source for the highly excited Rg** formation (reaction (1 0 )). Second part is a 10-mm long carbon channel reactor (CBCR) for the reaction (2 0 ) and (3). The basic structure of the CH source is similar to the previous one [11]. In the new CH source, the following modifications were carried out in order to enhance the collision efficiency of Rg** with carbon before the collisional quenching of Rg**. A stainless steel ring electrode (RE) was sandwiched by a pair of 2-mm thick carbon electrodes (CBE) with an aperture of 3-mm B. The Teflon channel reactor was replaced by the 10-mm long carbon channel reactor (CBCR) with a hole of 6-mm B. Discharge Part
VP
PH
CE
PV1
CI
Reaction Part BC
CR
REH CBE CBE
RE
CBCR
Rg
(-HV) PV2
(Grounded)
H2
Fig. 1. A schematic draws of CH beam source. PV1 pulsed valve for Rg; VP valve protector; PH Teflon valve protector holder; CE conical electrode 3-mm B; CI ceramic insulator; CBE a pair of 2-mm thick carbon electrodes with an aperture of 3-mm B; RE ring electrode 4-mm B; REH Teflon ring electrode holder; CBCR 10-mm long carbon channel reactor with a hole of 6-mm B; BC beam collimator 3-mm B; PV2 pulsed valve for H2.
125
The produced CH radicals were state-selected and focused into the focusing point at 150-cm downstream from the nozzle by a 80-cm long electrostatic hexapole field. The CH radicals were detected by the saturated LIF X2P transispectroscopy (SLIF) for R-branch in A2D tion. The SLIF signal was detected by a photomultiplier (Hamamatsu R1635P) mounted at 7-cm apart from the beam crossing point and the signal was counted by a photon-counter (Stanford SR400) with the gate width of 1.0 ls after the delay time of 0.02 ls from the laser irradiation. The density of CH was estimated from the SLIF intensity. As an application of the present CH beam source to the reaction dynamics, the state-selected CH were focused into the reaction cell filled with O2 gas. The detail of the procedure was shown in the previous paper [12]. The OH(A) chemiluminescence from the reaction of CH + O2 was detected by a photomultiplier (Hamamatsu R1635P) through a color filter (HOYA U330). 3. Results and discussion 3.1. CH beam source The newly developed CH beam source was optimized for the operation conditions; DC discharge conditions (stagnation pressure of Rg, delay time from Rg injection, voltage, pulse width), H2 gas conditions (stagnation pressure, delay time from Rg injection). The optimized operation conditions were summarized in Table 1. The conditions are extremely different from those for the previous CH source [11]. In particular, a relatively lower stagnation pressure of Rg is favorable for the present source. It is just conceivable that the higher pressure of Rg is desirable in the previous CH source for the formation of metastable Rg* by the collisional quenching from the higher excited states. On the contrary, an excess pressure gives the undesirable effects in the new CH source because of the acceleration of the collisional deactivation of the highly excited Rg** state to the lowest one. Fig. 2 shows the time profiles of the focused CH beam at the optimum condition with the flight length of 150 cm, which were measured using SLIF by scanning the delay time of the laser irradiation from the time origin (DC discharge). As compared with the pulse width for DC discharge, the beam profile is rather narrow, indicating that the CH formation occurs at only a part of the discharge period. From the trajectory simulation of the focusing curve [15], the stream velocity was roughly determined to be 1900 m s1, 1000 m s1, 650 m s1 for He, Ne, Ar, respectively. In addition, the more efficient translational cooling was recognized for the present CH beam as compared with the previous one. This might reflect the efficient pickup of CH radical by the seeding gas without CO gas. The rotational state distribution of the CH beam was roughly characterized by the rotational temperature of 60 K for every Rg on the basis of the relative LIF intensity of each rotational state. The CH beam intensities are
126
Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
Table 1 Optimum conditions for the newly developed CH source H2 gas conditions He Ne Ar Previousb a b
Discharge conditions a
Pressure (Torr)
Delay time (ms)
Pressure of Rg (Torr)
Voltage (V)
Delaya time (ms)
Pulse width (ms)
60 30 30 140 (CO/H2)
0.2 0.1 0.1 1.0
1000 1000 1200 1800
1600 2000 1700 1900
0.3 0.6 0.5 0.8
0.6 0.6 0.7 0.45
Time origin is the trigger for Rg injection. Ref. [11].
He
Ne
Ar
Relative CH Intensity / arb. units
CH Intensity / arb. units
He
1.0
1.0
0.5
0.5
Ne
Discharge
Ar 0 0
0 0
2.0
1.0
3.0
Time / ms Fig. 2. Time profile of CH beam under the optimum condition for each Rg gas obtained as the SLIF signals with the flight length of 150 cm.
significantly improved as compared with that for the previous CH source by the factor of 2 for He, and more than 1– 2 orders for Ne and Ar. 3.2. Rg dependence of CH intensity The relative CH density for three kinds of Rg is determined to be 1:0.22:0.06 for He:Ne:Ar. They were summarized in Fig. 3. Based on the heat of sublimation of carbon rod (7.41 eV) and the excitation energy of Rg** ðERg Þ, the excess energy of the carbon atom (EExcess) sublimated in the reaction of Rg** + C (solid) ! C + Rg is expressed by EExcess ¼ ERg 7:41 eV. If the some portion of the excess energy, aRg EExcess , can be statistically converted into the translational and the electronic excitation energy of the sublimated carbon atom,
5
10
15
20
Excess Energy EExcess / eV Fig. 3. Correlation of the relative CH densities under three kinds of Rg (He, Ne, Ar) (horizontal solid lines) with the excess energy in the reaction (2 0 ). Excess energy for each metastable Rg* (open circles). Typical excess energy for the highly excited Rg** states characterized by the transitions to 3s orbital for He, to 3p orbital for Ne, to 4p orbital for Ar (squares).
the relative population of C(1D) having the excitation energy of 1.26 eV can be deduced by the following Boltzmann factor: expð1:26=aRg EExcess Þ=b1 þ expð1:26=aRg EExcess Þc The experimental relative CH intensity fortuitously seems to have a good correlation with the relationship using the identical value of a = 0.03–0.04 (solid line) for three rare gases, although it is difficult to identify the states of Rg** that are favorable for C(1D) formation. 3.3. The CH beam intensity and focusing curves Under the optimized beam condition, we measured the focusing curve of each jJ, Fi, Mæ state and determined the absolute CH density.
Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
Fig. 4 shows the focusing curves measured for the jJ, Fi, M > = j1/2, F2, 1/2æ states without and with the beam stop for three kinds of Rg: He, Ne, Ar. The focusing curves without the beam stop showed a distinctive feature of long asymmetric tail to higher voltage that is due to the K-doubling coupling effect. For the application of the stateselected CH beam, it is very important to achieve a good resolution of the state-selection. From the comparison of a pair of focusing curves, at a glance, it is found that the beam stop clearly remove the asymmetric feature in the focusing curve and greatly improve the resolution of the focusing curve. The absolute CH beam density was
(a)
Intensity / 107 cm-3
without BS
Rg = He
As a demonstration of the usefulness of the present CH beam source, CH (X2P) + O2 reaction was studied with the beam stop under the Ne seeded condition (i.e. at the lower collision energy of 0.06 eV). In the previous study [11,12], no rotational state dependence of the reaction cross-section of OH (A) formation was observed at the collision energy of 0.14 eV. Fig. 5 shows the dependence of OH (A) chemiluminescence intensity on the hexapole voltage (OH (A) CL-focusing curve) at the collision energy of 0.06 eV. The OH (A) CL-focusing curve showed the clear structure owing to the higher resolution of CH focusing curve in the present study. The OH (A) formation in the CH (X2P) + O2 reaction was confirmed again because the clear signal enhancement of OH (A) chemiluminescence by applying the hexapole field directly indicates that this chemiluminescence is attributed to the reaction with pure CH. In order
3 with BS
2 1
0 1.2 (b) Rg = Ne
Intensity / 107 cm-3
determined from the SLIF intensity in a manner similar to that described in the previous study [11]. The maximum CH flux intensity for the focused jJ, Fi, M æ = j1/2, F2, 1/2æ state without the beam stop reached to (8.8 ± 0.4) · 1012, (1.0 ± 0.1) · 1012, (1.8 ± 0.2) · 1011 radicals cm2 s1 at 150 cm downstream from the nozzle for the discharge using He, Ne, Ar, respectively. The CH flux intensity for the jJ, Fi æ = j1/2, F2æ state before state-selection is estimated to be (1.3 ± 0.1) · 1016 radicals sr1 s1 from the direct beam intensity (beam intensity at hexapole voltage off) by assuming 1/z2 dependency, where z is the distance from the nozzle. 3.4. Application to the CH (X2P) + O2 ! OH (A) + CO reaction at the collision energy of 0.06 eV
5 4
127
without BS
0.8
0.4 with BS
30
0 20 3
Intensity / arb. units
(c) Rg = Ar
Intensity / 106 cm-3
without BS 2
10
1 0
with BS
0
0
2
4
6
8
10
Hexapole Voltage / kV 0
2
4 6 Hexapole Voltage / kV
8
10
Fig. 4. Focusing curves for the jJ, Fi, M i ¼j 1=2, F2, 1/2æ states without . (open circle) and with the beam stop (filled circle) measured by LIF under three kinds of Rg: (a) He; (b) Ne; and (c) Ar.
Fig. 5. OH (A) CL-focusing curve with the beam stop at the collision energy of 0.06 eV. Experimental (filled circle), estimated OH (A) CLfocusing curve with no CH rotational state dependence (dashed line), and simulated OH (A) CL-focusing curve with the rotational state dependence of the cross section (solid line).
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Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
to compare the present result with the previous one, the OH (A) CL-focusing curve was estimated for the case that the reaction cross-section has no dependence on the CH rotational state. The estimated OH (A) CL-focusing curve corresponds to the overall CH intensity as the sum of 6 focusing curves measured by SLIF for the 6 rotational jJ, Fiæ states that are focusable within the hexapole voltage up to 10 kV. It was shown in Fig. 5 as a dashed line. At a glance, a noticeable difference is recognized between the experimental OH (A) CL-focusing curve and the estimated one (dashed line). This difference indicates that the reaction cross-section of OH (A) formation has some CH rotational state dependence at the collision energy of 0.06 eV, in contrast with no dependence at the collision energy of 0.14 eV. The experimental OH (A) CL-focusing curve can be nicely reproduced by taking into account the rotational state dependence of the cross section. A solid line in Fig. 5 is the simulated OH (A) CL-focusing curve that was obtained as the overall CH intensity by summing up the 6 CH focusing curves weighted by the rotational state dependent cross section in the same manner described in the previous study [12]. At the collision energy of 0.06 eV, the resultant relative cross sections for each jJ, Fiæ rotational state are determined to be rðj 1=2; F 2 iÞ : rðj 3=2; F 1 iÞ : rðj 3=2; F 2 iÞ : rðj 5=2; F 1 iÞ : rðj 5=2; F 2 iÞ : rðj 7=2; F 1 iÞ ¼ 1:2 : 1:0 : 2:0 : 3:0 : 1:0 : 1:0. In the previous study, the cross section for the OH (A) formation channel was determined to be (1.5 ± 0.1) · 1019 cm2. A preliminary analysis of the cross section roughly indicates that the cross section of OH (A) formation has a positive dependence on the collision energy as a whole in contrast with the negative temperature depen-
dence of the total reaction cross section of the CH + O2 reaction [16]. By using the present CH beam source, we can easily control the CH beam velocity by changing the mixing ratio of two kinds of Rg. The further study on the collision energy dependent reaction dynamics of the state-selected CH + O2 reaction is now in progress. References [1] S.S. Wagel, T. Carrington, S.V. Filseth, C.M. Sadowski, Chem. Phys. 69 (1982) 61. [2] D.A. Lichtin, M.R. Breman, M.C. Lin, Chem. Phys. Lett. 108 (1984) 18. [3] J.D. Anthony, K.H. Ronald, T.B. Craig, J. Phys. Chem. 95 (1991) 3180. [4] S. Okada, K. Yamasaki, H. Matsui, K. Saito, K. Okada, Bull. Chem. Soc. Jpn. 66 (1993) 1004. [5] P. Bocherel, L.B. Herebert, B.R. Rowe, I.R. Sims, I.W.M. Smith, D. Travers, J. Phys. Chem. 100 (1996) 3063. [6] S.A. Carl, M.V. Poppel, J. Peeters, J. Phys. Chem. A 107 (2003) 11001. [7] A. Bergeat, T. calvo, N. Daugey, J.-C. Loison, G. Dorthe, J. Phys. Chem. A 102 (1998) 8124. [8] N. Daugey, A. Bergeat, A. Schuck, P. Caubet, G. Dorthe, Chem. Phys. 87 (1997) 222. [9] M.A. Weibel, T.D. Hain, T.J. Curtiss, J. Chem. Phys. 108 (1998) 3134. [10] K.H. Becker, B. Engelhardt, H. Geiger, R. Kurtenbach, G. Schrey, P. Wiesen, Chem. Phys. Lett. 195 (1992) 322. [11] K. Ikejiri, H. Ohoyama, Y. Nagamachi, T. Kasai, Chem. Phys. Lett. 401 (2005) 465. [12] Y. Nagamachi, H. Ohoyama, K. Ikejiri, T. Kasai, J. Chem. Phys. 122 (2005) 064307. [13] M. Zachwieja, J. Mol. Spectrosc. 170 (1995) 285. [14] E.L. Hill, J.H. Van Vleck, Phys. Rev. 32 (1923) 250. [15] H. Ohoyama, Y. Nagamachi, T. Kasai, Stereodynamics of Chemical Reactions, Eur. Phys. J. D (special issue), in press. [16] B. Pascal, B.H. Lee, R.R. Bertrand, R.S. Ian, W.M.S. Ian, T. Daniel, J. Phys. Chem. 100 (1996) 3063.
Development of a highly intense state-selected CH radical beam source for the study on the collision energy dependent reaction dynamics Y. Nagamachi, H. Ohoyama *, K. Yamakawa, T. Kasai Department of Chemistry, Graduate School of Science, Osaka University, Machikaneyama-chyo-1-1, Toyonaka, Osaka 560-0043, Japan Received 16 December 2005; in final form 10 January 2006 Available online 8 February 2006
Abstract A velocity-variable highly intense pulsed supersonic CH (X2P) radical beam source was newly developed for the study on the collision energy dependent reaction dynamics of the state-selected CH radical. CH radicals were state-selected by an electrostatic hexapole field. The focusing curves and the CH flux intensities for the jJ, Fi, Mæ states were measured by a saturated laser-induced fluorescence spectroscopy. As a demonstration, the rotational state dependence of the reaction cross section of CH + O2 ! OH(A) + CO reaction was determined at the collision energy of 0.06 eV. 2006 Elsevier B.V. All rights reserved.
1. Introduction
[12]. In the previous CH source, the following reaction schemes were adopted to produce CH radicals.
Methylidyne radical (CH) is one of the most important intermediates in many fields including combustion, atmospheric, interstellar. From this reason, a number of studies on its reaction rates have been carried out [1–6]. In contrast with its importance, however, the studies on its reaction dynamics have been extremely limited [7]. Surprisingly, even for most of the fundamental reaction systems, the reaction products have not been identified. This is because the radical source always becomes mixture of reactive species in the process of CH radical generation [8–10]. From theoretical and practical importance of the CH reaction, it is worthwhile to develop a high-intensity, high-purity state-selected CH radical beam source. Recently, we succeeded in developing a highly intense and pure CH radical beam source using an electrostatic hexapole field [11], and applied it to the elementary reaction of CH + NO, O2 at the collision energy of 0.14 eV
He ! He ð23 SÞ
*
Corresponding author. Fax: +81 6 6850 5403. E-mail address: [email protected] (H. Ohoyama).
0009-2614/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.01.025
3
ð1Þ 1
þ
1
He ð2 SÞ þ COðX R Þ ! Cð DÞ þ O þ He 1
2
Cð DÞ þ H2 ! CHðX P; v ¼ 0Þ
ð2Þ ð3Þ
Although this CH source can produce a high-intensity and high-purity state-selected CH radical beam, the source has a disadvantage that the velocity of CH beam is not variable by changing seeding gas, because of the extremely poor efficiency of CH formation by using other rare gas except for He. For the better understanding of the reaction dynamics, it is very important to control the collision energy. In the present study, we developed a velocity-variable highly intense CH radical beam source. The CH beam was characterized by a saturated laser-induced fluorescence spectroscopy (SLIF) in the A2D X2P transition [13]. The maximum flux intensities of the CH radical in each rotational jJ, Fi, Mæ states, were determined by using SLIF, where J is the total angular momentum. M is the projection of J on the quantization axis and Fi designates the fine state [14]. As a demonstration of the usefulness of the velocity-variable CH beam source, CH (X2P) + O2 reaction was studied at the collision energy of 0.06 eV.
Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
2. Experimental The experimental setup is almost same with the previous one except for the CH beam source [11,12]. The inefficient CH formation in the previous CH source by using other rare gas except for He should be caused by the extremely inefficient C(1D) formation in the reaction (2). The efficiency of C(1D) formation using Ne and Ar was estimated to be at least 2–3 order lower than that using He (i.e. out of the detection limit by SLIF). In order to overcome this disadvantage, a new approach using the reactions described below was carried out. Rg ! Rg
ð10 Þ
Rg þ CðsolidÞ ! Cð1 DÞ þ Rg
ð20 Þ
Cð1 DÞ þ H2 ! CHðX2 P; v ¼ 0Þ
ð3Þ
In the newly developed CH source, C(1D) is generated by the collision of solid carbon with the highly excited states of rare gas, Rg**. Based on the heat of sublimation of solid carbon (7.41 eV) and the excitation energy of C (1D) (1.26 eV), the threshold for the reaction (2 0 ) is estimated to be 8.67 eV. Therefore, the formation of C(1D) by Ne** and Ar** is energetically possible. Fig. 1 shows a schematic view of the CH beam source we have newly developed. The beam source consists of two parts. First part is a discharge source for the highly excited Rg** formation (reaction (1 0 )). Second part is a 10-mm long carbon channel reactor (CBCR) for the reaction (2 0 ) and (3). The basic structure of the CH source is similar to the previous one [11]. In the new CH source, the following modifications were carried out in order to enhance the collision efficiency of Rg** with carbon before the collisional quenching of Rg**. A stainless steel ring electrode (RE) was sandwiched by a pair of 2-mm thick carbon electrodes (CBE) with an aperture of 3-mm B. The Teflon channel reactor was replaced by the 10-mm long carbon channel reactor (CBCR) with a hole of 6-mm B. Discharge Part
VP
PH
CE
PV1
CI
Reaction Part BC
CR
REH CBE CBE
RE
CBCR
Rg
(-HV) PV2
(Grounded)
H2
Fig. 1. A schematic draws of CH beam source. PV1 pulsed valve for Rg; VP valve protector; PH Teflon valve protector holder; CE conical electrode 3-mm B; CI ceramic insulator; CBE a pair of 2-mm thick carbon electrodes with an aperture of 3-mm B; RE ring electrode 4-mm B; REH Teflon ring electrode holder; CBCR 10-mm long carbon channel reactor with a hole of 6-mm B; BC beam collimator 3-mm B; PV2 pulsed valve for H2.
125
The produced CH radicals were state-selected and focused into the focusing point at 150-cm downstream from the nozzle by a 80-cm long electrostatic hexapole field. The CH radicals were detected by the saturated LIF X2P transispectroscopy (SLIF) for R-branch in A2D tion. The SLIF signal was detected by a photomultiplier (Hamamatsu R1635P) mounted at 7-cm apart from the beam crossing point and the signal was counted by a photon-counter (Stanford SR400) with the gate width of 1.0 ls after the delay time of 0.02 ls from the laser irradiation. The density of CH was estimated from the SLIF intensity. As an application of the present CH beam source to the reaction dynamics, the state-selected CH were focused into the reaction cell filled with O2 gas. The detail of the procedure was shown in the previous paper [12]. The OH(A) chemiluminescence from the reaction of CH + O2 was detected by a photomultiplier (Hamamatsu R1635P) through a color filter (HOYA U330). 3. Results and discussion 3.1. CH beam source The newly developed CH beam source was optimized for the operation conditions; DC discharge conditions (stagnation pressure of Rg, delay time from Rg injection, voltage, pulse width), H2 gas conditions (stagnation pressure, delay time from Rg injection). The optimized operation conditions were summarized in Table 1. The conditions are extremely different from those for the previous CH source [11]. In particular, a relatively lower stagnation pressure of Rg is favorable for the present source. It is just conceivable that the higher pressure of Rg is desirable in the previous CH source for the formation of metastable Rg* by the collisional quenching from the higher excited states. On the contrary, an excess pressure gives the undesirable effects in the new CH source because of the acceleration of the collisional deactivation of the highly excited Rg** state to the lowest one. Fig. 2 shows the time profiles of the focused CH beam at the optimum condition with the flight length of 150 cm, which were measured using SLIF by scanning the delay time of the laser irradiation from the time origin (DC discharge). As compared with the pulse width for DC discharge, the beam profile is rather narrow, indicating that the CH formation occurs at only a part of the discharge period. From the trajectory simulation of the focusing curve [15], the stream velocity was roughly determined to be 1900 m s1, 1000 m s1, 650 m s1 for He, Ne, Ar, respectively. In addition, the more efficient translational cooling was recognized for the present CH beam as compared with the previous one. This might reflect the efficient pickup of CH radical by the seeding gas without CO gas. The rotational state distribution of the CH beam was roughly characterized by the rotational temperature of 60 K for every Rg on the basis of the relative LIF intensity of each rotational state. The CH beam intensities are
126
Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
Table 1 Optimum conditions for the newly developed CH source H2 gas conditions He Ne Ar Previousb a b
Discharge conditions a
Pressure (Torr)
Delay time (ms)
Pressure of Rg (Torr)
Voltage (V)
Delaya time (ms)
Pulse width (ms)
60 30 30 140 (CO/H2)
0.2 0.1 0.1 1.0
1000 1000 1200 1800
1600 2000 1700 1900
0.3 0.6 0.5 0.8
0.6 0.6 0.7 0.45
Time origin is the trigger for Rg injection. Ref. [11].
He
Ne
Ar
Relative CH Intensity / arb. units
CH Intensity / arb. units
He
1.0
1.0
0.5
0.5
Ne
Discharge
Ar 0 0
0 0
2.0
1.0
3.0
Time / ms Fig. 2. Time profile of CH beam under the optimum condition for each Rg gas obtained as the SLIF signals with the flight length of 150 cm.
significantly improved as compared with that for the previous CH source by the factor of 2 for He, and more than 1– 2 orders for Ne and Ar. 3.2. Rg dependence of CH intensity The relative CH density for three kinds of Rg is determined to be 1:0.22:0.06 for He:Ne:Ar. They were summarized in Fig. 3. Based on the heat of sublimation of carbon rod (7.41 eV) and the excitation energy of Rg** ðERg Þ, the excess energy of the carbon atom (EExcess) sublimated in the reaction of Rg** + C (solid) ! C + Rg is expressed by EExcess ¼ ERg 7:41 eV. If the some portion of the excess energy, aRg EExcess , can be statistically converted into the translational and the electronic excitation energy of the sublimated carbon atom,
5
10
15
20
Excess Energy EExcess / eV Fig. 3. Correlation of the relative CH densities under three kinds of Rg (He, Ne, Ar) (horizontal solid lines) with the excess energy in the reaction (2 0 ). Excess energy for each metastable Rg* (open circles). Typical excess energy for the highly excited Rg** states characterized by the transitions to 3s orbital for He, to 3p orbital for Ne, to 4p orbital for Ar (squares).
the relative population of C(1D) having the excitation energy of 1.26 eV can be deduced by the following Boltzmann factor: expð1:26=aRg EExcess Þ=b1 þ expð1:26=aRg EExcess Þc The experimental relative CH intensity fortuitously seems to have a good correlation with the relationship using the identical value of a = 0.03–0.04 (solid line) for three rare gases, although it is difficult to identify the states of Rg** that are favorable for C(1D) formation. 3.3. The CH beam intensity and focusing curves Under the optimized beam condition, we measured the focusing curve of each jJ, Fi, Mæ state and determined the absolute CH density.
Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
Fig. 4 shows the focusing curves measured for the jJ, Fi, M > = j1/2, F2, 1/2æ states without and with the beam stop for three kinds of Rg: He, Ne, Ar. The focusing curves without the beam stop showed a distinctive feature of long asymmetric tail to higher voltage that is due to the K-doubling coupling effect. For the application of the stateselected CH beam, it is very important to achieve a good resolution of the state-selection. From the comparison of a pair of focusing curves, at a glance, it is found that the beam stop clearly remove the asymmetric feature in the focusing curve and greatly improve the resolution of the focusing curve. The absolute CH beam density was
(a)
Intensity / 107 cm-3
without BS
Rg = He
As a demonstration of the usefulness of the present CH beam source, CH (X2P) + O2 reaction was studied with the beam stop under the Ne seeded condition (i.e. at the lower collision energy of 0.06 eV). In the previous study [11,12], no rotational state dependence of the reaction cross-section of OH (A) formation was observed at the collision energy of 0.14 eV. Fig. 5 shows the dependence of OH (A) chemiluminescence intensity on the hexapole voltage (OH (A) CL-focusing curve) at the collision energy of 0.06 eV. The OH (A) CL-focusing curve showed the clear structure owing to the higher resolution of CH focusing curve in the present study. The OH (A) formation in the CH (X2P) + O2 reaction was confirmed again because the clear signal enhancement of OH (A) chemiluminescence by applying the hexapole field directly indicates that this chemiluminescence is attributed to the reaction with pure CH. In order
3 with BS
2 1
0 1.2 (b) Rg = Ne
Intensity / 107 cm-3
determined from the SLIF intensity in a manner similar to that described in the previous study [11]. The maximum CH flux intensity for the focused jJ, Fi, M æ = j1/2, F2, 1/2æ state without the beam stop reached to (8.8 ± 0.4) · 1012, (1.0 ± 0.1) · 1012, (1.8 ± 0.2) · 1011 radicals cm2 s1 at 150 cm downstream from the nozzle for the discharge using He, Ne, Ar, respectively. The CH flux intensity for the jJ, Fi æ = j1/2, F2æ state before state-selection is estimated to be (1.3 ± 0.1) · 1016 radicals sr1 s1 from the direct beam intensity (beam intensity at hexapole voltage off) by assuming 1/z2 dependency, where z is the distance from the nozzle. 3.4. Application to the CH (X2P) + O2 ! OH (A) + CO reaction at the collision energy of 0.06 eV
5 4
127
without BS
0.8
0.4 with BS
30
0 20 3
Intensity / arb. units
(c) Rg = Ar
Intensity / 106 cm-3
without BS 2
10
1 0
with BS
0
0
2
4
6
8
10
Hexapole Voltage / kV 0
2
4 6 Hexapole Voltage / kV
8
10
Fig. 4. Focusing curves for the jJ, Fi, M i ¼j 1=2, F2, 1/2æ states without . (open circle) and with the beam stop (filled circle) measured by LIF under three kinds of Rg: (a) He; (b) Ne; and (c) Ar.
Fig. 5. OH (A) CL-focusing curve with the beam stop at the collision energy of 0.06 eV. Experimental (filled circle), estimated OH (A) CLfocusing curve with no CH rotational state dependence (dashed line), and simulated OH (A) CL-focusing curve with the rotational state dependence of the cross section (solid line).
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Y. Nagamachi et al. / Chemical Physics Letters 421 (2006) 124–128
to compare the present result with the previous one, the OH (A) CL-focusing curve was estimated for the case that the reaction cross-section has no dependence on the CH rotational state. The estimated OH (A) CL-focusing curve corresponds to the overall CH intensity as the sum of 6 focusing curves measured by SLIF for the 6 rotational jJ, Fiæ states that are focusable within the hexapole voltage up to 10 kV. It was shown in Fig. 5 as a dashed line. At a glance, a noticeable difference is recognized between the experimental OH (A) CL-focusing curve and the estimated one (dashed line). This difference indicates that the reaction cross-section of OH (A) formation has some CH rotational state dependence at the collision energy of 0.06 eV, in contrast with no dependence at the collision energy of 0.14 eV. The experimental OH (A) CL-focusing curve can be nicely reproduced by taking into account the rotational state dependence of the cross section. A solid line in Fig. 5 is the simulated OH (A) CL-focusing curve that was obtained as the overall CH intensity by summing up the 6 CH focusing curves weighted by the rotational state dependent cross section in the same manner described in the previous study [12]. At the collision energy of 0.06 eV, the resultant relative cross sections for each jJ, Fiæ rotational state are determined to be rðj 1=2; F 2 iÞ : rðj 3=2; F 1 iÞ : rðj 3=2; F 2 iÞ : rðj 5=2; F 1 iÞ : rðj 5=2; F 2 iÞ : rðj 7=2; F 1 iÞ ¼ 1:2 : 1:0 : 2:0 : 3:0 : 1:0 : 1:0. In the previous study, the cross section for the OH (A) formation channel was determined to be (1.5 ± 0.1) · 1019 cm2. A preliminary analysis of the cross section roughly indicates that the cross section of OH (A) formation has a positive dependence on the collision energy as a whole in contrast with the negative temperature depen-
dence of the total reaction cross section of the CH + O2 reaction [16]. By using the present CH beam source, we can easily control the CH beam velocity by changing the mixing ratio of two kinds of Rg. The further study on the collision energy dependent reaction dynamics of the state-selected CH + O2 reaction is now in progress. References [1] S.S. Wagel, T. Carrington, S.V. Filseth, C.M. Sadowski, Chem. Phys. 69 (1982) 61. [2] D.A. Lichtin, M.R. Breman, M.C. Lin, Chem. Phys. Lett. 108 (1984) 18. [3] J.D. Anthony, K.H. Ronald, T.B. Craig, J. Phys. Chem. 95 (1991) 3180. [4] S. Okada, K. Yamasaki, H. Matsui, K. Saito, K. Okada, Bull. Chem. Soc. Jpn. 66 (1993) 1004. [5] P. Bocherel, L.B. Herebert, B.R. Rowe, I.R. Sims, I.W.M. Smith, D. Travers, J. Phys. Chem. 100 (1996) 3063. [6] S.A. Carl, M.V. Poppel, J. Peeters, J. Phys. Chem. A 107 (2003) 11001. [7] A. Bergeat, T. calvo, N. Daugey, J.-C. Loison, G. Dorthe, J. Phys. Chem. A 102 (1998) 8124. [8] N. Daugey, A. Bergeat, A. Schuck, P. Caubet, G. Dorthe, Chem. Phys. 87 (1997) 222. [9] M.A. Weibel, T.D. Hain, T.J. Curtiss, J. Chem. Phys. 108 (1998) 3134. [10] K.H. Becker, B. Engelhardt, H. Geiger, R. Kurtenbach, G. Schrey, P. Wiesen, Chem. Phys. Lett. 195 (1992) 322. [11] K. Ikejiri, H. Ohoyama, Y. Nagamachi, T. Kasai, Chem. Phys. Lett. 401 (2005) 465. [12] Y. Nagamachi, H. Ohoyama, K. Ikejiri, T. Kasai, J. Chem. Phys. 122 (2005) 064307. [13] M. Zachwieja, J. Mol. Spectrosc. 170 (1995) 285. [14] E.L. Hill, J.H. Van Vleck, Phys. Rev. 32 (1923) 250. [15] H. Ohoyama, Y. Nagamachi, T. Kasai, Stereodynamics of Chemical Reactions, Eur. Phys. J. D (special issue), in press. [16] B. Pascal, B.H. Lee, R.R. Bertrand, R.S. Ian, W.M.S. Ian, T. Daniel, J. Phys. Chem. 100 (1996) 3063.