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Hydrogen migration in acetonitrile in intense laser fields in competition with two-body Coulomb explosion
Journal of Electron Spectroscopy and Related Phenomena 141 (2004) 195–200
Hydrogen migration in acetonitrile in intense laser fields in competition with two-body Coulomb explosion Akiyoshi Hishikawaa , Hirokazu Hasegawab , Kaoru Yamanouchib,∗ b
a Institute for Molecular Science, National Institutes of Natural Science, Myodaiji, Okazaki, Aichi 444-8585, Japan Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 10 May 2004; received in revised form 22 June 2004; accepted 22 June 2004 Available online 7 October 2004
Abstract Two-body Coulomb explosion processes of acetonitrile (CH3 CN) and deuterated acetonitrile (CD3 CN), CH3 CN2+ → CH3−n + + Hn CN+ and CD3 CN2+ → CD3−n + + Dn CN+ (n = 0–2), in an intense laser field (0.15 PW/cm2 , 70 fs) are investigated by the coincidence momentum imaging method. The comparable yields derived for the three pathways (n = 0–2) shows that the hydrogen atom migration proceeds in competition with the Coulomb explosion. The angular distributions of the fragment ions for n = 0 exhibits a sharp peak along the laser polarization direction while the angular distribution becomes more isotropic as n increases. Based on a least-squares analysis of the fragment anisotropy, the dissociation lifetimes of the doubly charged acetonitrile were determined, from which the time scale of the hydrogen migration as well as the deformation of the C–C–N skeleton prior to the explosion were discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Intense laser fields; Coulomb explosion; Hydrogen migration; Coincidence momentum imaging; Acetonitrile.
1. Introduction Dynamics of molecules exposed to an intense laser field (∼1 PW/cm2 ) whose magnitude is comparable with the internal Coulomb field has been an attractive research target in the last decade [1]. In our earlier studies, on the basis of the anisotropic distribution of the atomic fragment ion ejected by the Coulomb explosion, ultrafast skeletal deformation processes of triatomic molecules such as CO2 [2] and CS2 [3] in an intense laser field were investigated. In order to know more detailed dynamics of molecules in an intense laser field, we introduced recently a method called coincidence momentum imaging (CMI) of fragments ions for the first time [4]. This approach has two major advantages over the previous momentum imaging methods such as the mass-resolved momentum imaging [2,3] and the covariance mapping [5]. First, all the fragment ions originating from a single parent ion are ∗
Corresponding author. Tel.: +81 3 5841 4334; fax: +81 3 5689 7347. E-mail address: [email protected] (K. Yamanouchi).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.06.009
identified in coincidence so that the charge number of the parent ions as well as its dissociation pathways can be specified definitively. Second, the three-dimensional (3D) momentum vectors of respective fragment ions are determined in the laboratory frame for every single event of the Coulomb explosion. Indeed, based on the correlation between the determined fragment momentum vectors, it became possible to identify the existence of sequential and non-sequential threebody explosion pathways [6] and to discuss the evolution of a nuclear wavepacket on the light-dressed potential energy surface (PES) [7] for CS2 in an intense laser field. In the present study, the two-body Coulomb explosion processes of acetonitrile (CH3 CN) [8] and deuterated acetonitrile (CD3 CN), CH3 CN2+ → CH3−n + + Hn CN+ (n = 0 − 2),
(1)
CD3 CN2+ → CH3−n + + Dn CN+ (n = 0 − 2),
(2)
in an ultrafast intense laser field (0.15 PW/cm2 , 70 fs) are investigated by the CMI method, where an ultrafast nuclear
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rearrangement process called hydrogen migration [9] occurring prior to the explosion process is identified. The fragment anisotropy becomes more isotropic as n increases from 0 to 2, showing that the rate of the Coulomb explosion becomes comparable with or even longer than the rotational period of the parent molecule as the migration of hydrogen atoms from the methyl group to the nitrile group proceeds. A quantitative analysis of the fragment anisotropy and the branching ratio of the three Coulomb explosion pathways (n = 0–2), competition between the hydrogen atom migration in the intense laser field and the two-body Coulomb explosion process is investigated.
2. Experimental The details of the experimental setup have been described previously [8]. Briefly, ultrashort laser pulses (70 fs, 0.15 mJ/pulse, 3 kHz) were focused at the crossing point of a sample molecular beam and the laser beam in an ultrahigh vacuum chamber with a base pressure of ∼3 × 10−11 Torr. The sample vapor of acetonitrile at room temperature was introduced into the chamber as a skimmed effusive molecular beam. The fragment ions produced through the Coulomb explosion of CH3 CN or CD3 CN at the laser focal spot were guided to a position sensitive detector (PSD) with delay-line readouts [10,11] by three parallel electrodes in the velocity mapping configuration [4,12]. The direction of the laser polarization vector was set to be parallel to the detector plane. The momentum vector pi = (pi x , pi y , pi z ) of the i-th fragment ion with mass mi and charge qi was determined in the x–y–z laboratory frame from the flight time ti and the position (xi , yi ) on the PSD as a pair of the fragment ions whose momentum components are given in terms of the mass mi and the charge qi of the respective ions by, pxi = αx mi xi /ti, y
ions in the interaction region were excluded based on the momentum conservation rule, pi = 0.
3. Results and discussion 3.1. Fragmentation of acetonitrile Fig. 1(a) shows a part of the TOF mass spectrum of the ion species produced from CH3 CN in the intense laser field (0.15 PW/cm2 ). The peaks observed at the mass numbers m/z = 12, 13, and 28 can be unambiguously attributed to C+ , CH+ and H2 CN+ . On the other hand, two alternative assignments are possible for the respective peaks at m/z = 14, 15, 26, and 27, depending on whether the C C bond or the C N bond is broken in the Coulomb explosion process. For example, the peak at m/z = 15 may be assigned either to NH+ formed through the C N bond breaking after the migration of a hydrogen atom, or to CH3 + produced by the C C bond breaking with no hydrogen atom migration. A secure assignment of the fragment ions can be achieved from the comparison of the peak intensity pattern of CH3 CN with that of deuterated acetonitrile (CD3 CN) shown in Fig. 1(b). For example, two strong peak at m/z = 14 and 15 in Fig. 1(a) should correspond, respectively to the peaks at m/z = 16 and 18 in Fig. 1(b), showing that the peaks at m/z = 14 and 15 are CH2 + and CH3 + rather than N+ and NH+ . Similarly, the prominent peaks at m/z = 26 and 27 are assigned to CN+ and HCN+ , instead of C2 H2 + and C2 H3 + , respectively, suggesting that the fragmentation of acetonitrile in the intense laser field is dominated by the C C bond breaking.
(3)
pi = αy mi yi /ti,
(4)
pzi = αz qi ti (Urep − Uacc )/d,
(5)
where xi and yi are the shifts from the reference position in the x–y coordinate system where an ion released with px = py = 0 hits the detector, and ti is the time delay with respect to the reference flight time when an ion with pz = 0 reaches the detector, and Urep and Uacc represent the electric potentials of the repeller and the accelerator electrodes, respectively. A set of the correction factors, (αx , αy , αz ), in Eqs. (3)–(5) is introduced to account for a weak inhomogeniety in the electrostatic field in the ion extraction region of the TOF mass spectrometer in the velocity mapping configuration. False coincidence events originating from more than two parent
Fig. 1. The TOF spectra of CH3 CN (a) and CD3 CN (b) in intense laser fields (70 fs, 0.15 PW/cm2 ). From the comparison of the peak intensity patterns, the peaks observed in the mass range are securely assigned to the fragment ions produced after the C-C bond breaking (see text). Other small peaks originate from residual gases in the vacuum chamber.
A. Hishikawa et al. / Journal of Electron Spectroscopy and Related Phenomena 141 (2004) 195–200
3.2. Coincidence ion images Three different two-body Coulomb explosion pathways are identified as in Eq. (1), that is, the direct C C bond breaking without the migration of a hydrogen atom (n = 0) and the C C bond breaking after the single (n = 1) and double (n = 2) hydrogen migration. The coincidence ion images of the CH3−n + fragments produced through the three pathways are shown in Fig. 2(a)–(c). The n = 0 pathway exhibits a pair of clear crescent-like features, showing that the most of the fragment ions tend to be ejected in the laser polarization direction (ε). On the other hand, an anisotropy in the n = 1 pathway is less pronounced, and the nearly isotropic distribution is observed for the n = 2 pathway. This strong correlation between the fragment anisotropy and the extent of the hydrogen migration is also identified in the coincidence ion images in Figs. 2(d)–(f) for the two-body Coulomb explosion of the deuterated acetonitrile, CD3 CN2+ → CD3−n + + Dn CN+ (n = 0–2).
197
At the momentum value p ∼ 95 × 103 amu m/s where the peak is located in the momentum distributions for all the observed two-body pathways, the fractions (f) of the false events are estimated to be f < 0.03 [8], which secures that the observed coincidence events originate from the two-body explosion of single parent ions. On the other hand, weak features commonly identified at p ∼ 0 in Figs. 2(a)–(c) are attributed mostly to the false events (f ∼ 1) [8]. These false events are attributed to the fragmentation through neutral pathways, e.g., CH3 CN+ → CH3−n + + Hn CN or CH3−n + Hn CN+ (n = 0–2), in which either one of the two fragments is neutral. The relative yields of the respective explosion pathways can be derived from the number of coincidence events. For CH3 CN, the relative yields for the n = 1 and 2 pathways with respect to the n = 0 pathway are 1.3(1) and 1.0(1), respectively, while for CD3 CN the yield ratios with respect to the n = 0 pathway is 0.9(1) and 0.7(1) for the n = 1 and 2 pathways, respectively. The comparable yields for the three explosion pathways show that the migration of hydrogen atoms compete with the two-body Coulomb explosion of acetonitrile in the intense laser fields (0.15 PW/cm2 ). 3.3. Fragment anisotropy in Coulomb explosion of CH3 CN2+ and CD3 CN2+
Fig. 2. The two-dimensional coincidence ion images of (a) CH3 + , (b) CH2 + , (c) CH+ produced through the two-body Coulomb explosion of CH3 CN2+ and those of (d) CD3 + , (e) CD2 + , (f) CD+ produced from CD3 CN2+ . The weak central features commonly observed in these maps originate from the false coincidences. The anisotropy appearing in the thin circular distribution with respect to the direction of the laser polarization vector (ε) tends to become less pronounced as n increases from 0 to 2 reflecting the lifetime lengthening of the doubly charged parent ions after the hydrogen migration from the methyl group to the nitrile group.
In the CMI measurements, the momentum distribution of the fragment ions are determined in the three-dimensional momentum space so that their angular distribution, I(θ), with respect to the laser polarization direction can be derived in a straightforward manner without a complicated mathematical procedure such as an inverse Abel transformation. The angular distribution can be derived from a thin slice of the three-dimensional momentum distribution at pz ∼ 0 as shown in Fig. 3 for the three explosion pathways (n = 0–2) of CH3 CN and CD3 CN. It can be seen in these figures that the fragment ions tend to be ejected more isotropically as the hydrogen migration from the terminal carbon to the nitrile group proceeds. The extent of the anisotropy in the explosion pathways can be evaluated quantitatively by the expectation value of the squared cosine, = ʃI(θ)cos2 θ sinθ dθ/ʃI(θ) sinθ dθ. For CH3 CN, = 0.68 is obtained for the n = 0 pathway, while the angular distribution becomes more isotropic as n increases, i.e., = 0.49 and 0.37 for n = 1 and 2, respectively. A similar dependence of the fragment anisotropy on the explosion pathway is observed for CD3 CN, where = 0.71, 0.49, 0.39 for n = 0–2. 3.4. Dynamics extracted from the fragment anisotropy In the observed fragment anisotropy, both the information on the distribution of the principal molecular axis of the doubly charged acetonitrile prepared by the polarized laser field and that on the lifetime of the parent ion are encoded.
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Fig. 3. The angular distributions of the fragment ions for the two-body Coulomb explosion of CH3 CN2+ through the (a) n = 0, (b) n = 1, (c) n = 2 pathways and those of CD3 CN2+ through the (d) n = 0, (e) n = 1, (f) n = 2 pathways derived from a thin slice of the momentum vector distribution at pz ∼ 0. The results of the least-squares fit with the analytical expression (Eq. (6)) incorporating the effect of molecular rotation are shown for comparison (solid lines). The fit was improved for the n = 2 pathways ((c) and (f)), when the effect of off-axis fragment ejection in the molecular frame was taken into account (dashed lines).
As the lifetime becomes longer, the angular distribution of the fragment ions tends to become more isotropic due to the overall rotational motion of the parent molecules. It should also be noted that the extent of the fragment anisotropy could be lowered depending on the direction of the fragment recoil in the molecular frame. Unlike diatomic molecules, the recoil direction of the fragment ions from a polyatomic molecule is not always along the molecular principal axis. Incorporating these three factors mentioned above, i.e., (i) the angular distribution of the molecular axis of the parent molecules in the space fixed frame, Dmol (θ), (ii) the extent of the overall rotation of the parent molecules within a finite 0 of the dissociation lifetime, and (iii) the ejection angle of θm fragment ions with respect to the molecular axis of the parent molecule (see Fig. 4), the angular distribution of a fragment ion, I(θ s , ϕs ), can be expressed using a product of Legendre polynomials, Is (θs , ϕs ) =
1 1 0 aL cL PL (cos θm )PL (cos θs ), (6) 4π 2L+1 L
where the coefficients aL ’s are given by the Legendre expansion of Dmol (θ), π 2 aL = 4π (2L + 1) Dmol (θ)PL (cos θ) sin θ dθ. (7) 0
The coefficients cL reflecting the effect of the molecular rotation are expressed as cL = 4π2 (2L + 1)
π 0
Drot (θr )PL (cos θr ) sin θr dθr ,
(8)
Fig. 4. The coordinate systems describing the ejection direction of the fragment ions. The laser polarization direction is parallel with the Z-axis of the space-fixed X–Y–Z coordinate system. The direction of the z-axis of the molecular fixed coordinate system at the instance of the laser irradiation is defined by the polar angle θ, and the z-axis is transferred into the z -axis of another molecular fixed coordinate system by the rotation by the polar angle θ r . The fragment ejection direction (θ s , ϕs ) in the space fixed coordinate 0 in the second molecular fixed system is represented by the polar angle θm frame.
A. Hishikawa et al. / Journal of Electron Spectroscopy and Related Phenomena 141 (2004) 195–200
as a Legendre expansion of the probability Drot (θ r ) of finding a molecule dissociating after the rotation by an angle θ r in the molecular frame [13], Drot (θr ) =
1 4π2 τω
×
sin θr
exp(−(θr /τω)) + exp((θr − 2π)/τω) 1 − exp(−2π/τω)
, (9)
and are explicitly expressed as (L = 0) 1 L/2 cL = (1/τω)2 + (2k − 1)2 (L = 2, 4, . . .), (2L + 1) (1/τω)2 + (2k)2 k=1 (10) where τ and ω (=2π/τ rot ) represent, respectively the dissociation lifetime of a molecule and its rotational frequency. It is assumed that the explosion for the n = 0 pathway occurs immediately after the formation of the doubly charged acetonitrile (τω «1), and the fragments are ejected exclusively 0 = 0. This along the molecular axis, i.e., cL = 0 for L ≥ 2 and θm means that the observed sharp angular distribution for the n = 0 pathway represents Dmol (θ). Thus, the angular distribution of the fragments is expressed as Is (θs ) = Dmol (θs ) =
1 aL PL (cos θs ). 8π2
(11)
L
The results of the least-squares fit to Eq. (9) are shown in Fig. 3(a) and (d) for CH3 CN and CD3 CN, respectively, where the Legendre functions with L = 0, 2, 4, 6, 8 are used to achieve the convergence of the fit. With the five coefficients aL (L = 0, 2, 4, 6, 8) determined for the n = 0 pathway representing Dmol (θ), the least squares fit to Eq. (6) is performed for the n = 1 and 2 pathways by treating τω as a fitting parameter. The observed angular distributions for the n = 1 pathway for CH3 CN and CD3 CN are well reproduced when τω = 0.78(1) and 1.3(1), respectively, as shown in Fig. 3. Since the rotational periods, τ rot = 2B0 ( + 1), are τ rot = 2.4 ps and 2.2 ps at room temperature (300 K) for CH3 CN and CD3 CN, respectively, their dissociation lifetimes for the n = 1 pathway are calculated to be τ = 0.30 ps and 0.46 ps, respectively, where the expectation value of the rotational quantum number J, , is estimated to be = 22.5 and = 29.3 for CH3 CN and CD3 CN using the rotational constants of the ground vibrational state of B0 (CH3 CN) = 0.3068422247 cm−1 [14] and B0 (CD3 CN) = 0.262114119 cm−1 [15], respectively. On the other hand, for the n = 2 pathway, it is found that the observed angular distributions shown in Fig. 3(c) and (f) could not be described well by the simple model introduced above, that is, even the broadest distribution achieved by the model realized at the limit of τω » 1 was found to be substantially narrower than the observed distributions, suggesting the structural deformation of the C–C–N skeleton causes an
199
0 ( =0) off-axis ejection of the fragments at a non-zero angle θm 0 are adopted in the molecular frame. When both τω and θm as variable parameters, the observed distributions are fitted 0 = 29(4)◦ for CH CN and τω > well with τω > 10 and θm 3 0 = 22(4)◦ for CD CN. From the lower limit values 10 and θm 3 for τω, τ > 3.8 ps and τ > 3.5 ps are obtained for CH3 CN 0 , sugand CD3 CN, respectively. The large offset angles, θm gest that substantial skeletal structural deformation proceeds for the n = 2 pathway associated with the hydrogen migration at the doubly charged stage. Since an off-axis ejection of fragments is also expected to occur for the n = 1 pathway, the dissociation lifetimes determined above for n = 1 may be regarded as the lower limit values. As described in Section 3.2, the three two-body pathways (n = 0–2) have comparable yields for both normal and deuterated species. The relative yields for the n = 1 and 2 pathways with respect to the n = 0 pathway, Φ1 = 1.3(1) and Φ2 = 1.0(1), respectively, may reflect the ratio of the number of the doubly charged parent molecules prepared in the three different states from which dissociation proceeds along the n = 0–2 pathways, respectively. Alternatively, the branching ratios reflect the competition between the hydrogen migration and the Coulomb explosion, occurring after the doubly charged parent molecules are prepared in the common state. In the latter case, the migration of the first hydrogen atom from the methyl group to the nitrile group is considered to proceed about twice as fast as the direct Coulomb explosion process in the n = 0 pathway to account for sum of the relative yields, Φ1 + Φ2 = 2.3, for the n = 1 and 2 explosion pathways occurring after the hydrogen migration. The branching ratio, Φ1 /Φ2 = 1.3, may suggest that the migration of the second hydrogen atom proceeds on a time scale comparable with or slightly shorter than the dissociation lifetime of the n = 1 pathway, τ ∼ 0.30 ps. For CD3 CN, the relative yields of the n = 1 and 2 pathways, Φ1 = 0.9(1) and Φ2 = 0.7(1), are smaller than the corresponding values of CH3 CN, indicating that the migration rate of the hydrogen atoms from the methyl group to the nitrile group for CD3 CN is slower than that for CH3 CN, indicating that the latter model in which the dissociation and the hydrogen migration compete is more plausible. The sum of the relative yields, Φ1 + Φ2 = 1.6, for CD3 CN indicates that the migration rate of the first hydrogen atom is 1.6/2.3 = 0.7 times slower for CD3 CN than for CH3 CN, if the dissociation lifetime for the n = 0 pathway is assumed to be unchanged by the deuteration of the three hydrogen atoms. The migration of hydrogen atoms from the methyl group to the nitrile group as well as the C–C–N skeletal deformation may also occur in the singly charged stage of acetonitrile prior to the formation of the doubly charged acetonitrile if these processes could proceed within the time scale of the laser pulse duration. The contribution from these processes in the singly charged stage to the fragment anisotropy could be investigated by a pump-and-probe scheme with a probe laser whose pulse duration is much shorter than the pump laser pulse.
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4. Summary The two-body Coulomb explosion of acetonitrile (CH3 CN) and deuterated acetonitrile (CD3 CN) in an intense laser field (0.15 PW/cm2 , 70 fs) was investigated by the coincidence momentum imaging method. Three different explosion pathways for the respective species, i.e., CH3 CN2+ → CH3−n + + Hn CN+ (n = 0–2) and CD3 CN2+ → CD3−n + + Dn CN+ (n = 0–2), were securely identified, indicating the existence of the hydrogen migration process occurring prior to the two-body explosion. From the comparable yields of the three explosion pathways (n = 0–2), it was inferred that the migration of hydrogen atoms proceeds in competition with the two-body Coulomb explosion. The angular distribution of the fragment ions with respect to the laser polarization direction becomes more isotropic as the number of the migrated hydrogen atoms n increases. The least-squares fits of the observed anisotropic fragment distribution by a model in which (i) the initial angular distribution of the molecular axis just after the photoabsorption, (ii) the effect of molecular rotation of the parent molecule with a finite lifetime, and (iii) the off-axis ejection of fragment ions in the molecular frame are incorporated and the dissociation lifetimes of the doubly charged acetonitrile as well as the timescale of the hydrogen migration are determined. It was also inferred that the doubly charged parent molecules undergo the substantial skeletal deformation in intense laser fields along the C–C–N bending coordinate prior to the two body explosion through the n = 2 pathway as in the case of CO2 [16]. Acknowledgements The authors thank Ms. Kyoko Doi for her assistance for the present study. The present work was supported by the
CREST fund (1997–2001) from the Japan Science and Technology Corporation, the research grant from Graduate School of the University of Tokyo, and the Grant-in-Aid for Priority Areas for Control of Molecules in Intense Laser Fields and the Grant-in-Aid for the 21st Century COE Program for Frontiers in Fundamental Chemistry from Ministry of Education, Culture, Sports, Science and Technology.
References [1] K. Yamanouchi, Science 295 (2002) 1659. [2] A. Hishikawa, A. Iwamae, K. Yamanouchi, Phys. Rev. Lett. 83 (1999) 1127. [3] A. Iwasaki, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 346 (2001) 379. [4] H. Hasegawa, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 349 (2001) 57. [5] C. Cornaggia, Phys. Rev. A 54 (1996) 2555. [6] A. Hishikawa, H. Hasegawa, K. Yamanouchi, Chem. Phys. Lett. 361 (2002) 245. [7] A. Hishikawa, H. Hasegawa, K. Yamanouchi, Chem. Phys. Lett. 388 (2004) 1. [8] A. Hishikawa, H. Hasegawa, K. Yamanouchi, Phys. Scr. T110 (2004) 108. [9] H.F. Schaefer III, Acc. Chem. Res. 12 (1979) 288. [10] J. Ullrich, R. Moshammer, A. Dorn, R. D¨orner, L.P.H. Schmidt, H. Schmidt-B¨ocking, Rep. Prog. Phys. 66 (2003) 1463. [11] R. D¨orner, V. Mergel, O. Jagutzki, L. Spielberger, J. Ullrich, R. Moshammer, H. Schmidt-B¨ocking, Phys. Rep. 330 (2000) 95. [12] A.T.J.B. Eppink, D.H. Parker, Rev. Sci. Instrum. 68 (1997) 3477. [13] C. Jonah, J. Chem. Phys. 55 (1971) 1915. [14] M. Koivusaari, V.-M. Horneman, R. Anttila, J. Mol. Spec. 152 (1992) 377. [15] S. Alanko, M. Koivusaari, R. Paso, R. Anttila, J. Mol. Spec. 207 (2001) 89. [16] Y. Sato, H. Kono, S. Koseki, Y. Fujimura, J. Am. Chem. Soc. 125 (2003) 8019.
Hydrogen migration in acetonitrile in intense laser fields in competition with two-body Coulomb explosion Akiyoshi Hishikawaa , Hirokazu Hasegawab , Kaoru Yamanouchib,∗ b
a Institute for Molecular Science, National Institutes of Natural Science, Myodaiji, Okazaki, Aichi 444-8585, Japan Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Received 10 May 2004; received in revised form 22 June 2004; accepted 22 June 2004 Available online 7 October 2004
Abstract Two-body Coulomb explosion processes of acetonitrile (CH3 CN) and deuterated acetonitrile (CD3 CN), CH3 CN2+ → CH3−n + + Hn CN+ and CD3 CN2+ → CD3−n + + Dn CN+ (n = 0–2), in an intense laser field (0.15 PW/cm2 , 70 fs) are investigated by the coincidence momentum imaging method. The comparable yields derived for the three pathways (n = 0–2) shows that the hydrogen atom migration proceeds in competition with the Coulomb explosion. The angular distributions of the fragment ions for n = 0 exhibits a sharp peak along the laser polarization direction while the angular distribution becomes more isotropic as n increases. Based on a least-squares analysis of the fragment anisotropy, the dissociation lifetimes of the doubly charged acetonitrile were determined, from which the time scale of the hydrogen migration as well as the deformation of the C–C–N skeleton prior to the explosion were discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Intense laser fields; Coulomb explosion; Hydrogen migration; Coincidence momentum imaging; Acetonitrile.
1. Introduction Dynamics of molecules exposed to an intense laser field (∼1 PW/cm2 ) whose magnitude is comparable with the internal Coulomb field has been an attractive research target in the last decade [1]. In our earlier studies, on the basis of the anisotropic distribution of the atomic fragment ion ejected by the Coulomb explosion, ultrafast skeletal deformation processes of triatomic molecules such as CO2 [2] and CS2 [3] in an intense laser field were investigated. In order to know more detailed dynamics of molecules in an intense laser field, we introduced recently a method called coincidence momentum imaging (CMI) of fragments ions for the first time [4]. This approach has two major advantages over the previous momentum imaging methods such as the mass-resolved momentum imaging [2,3] and the covariance mapping [5]. First, all the fragment ions originating from a single parent ion are ∗
Corresponding author. Tel.: +81 3 5841 4334; fax: +81 3 5689 7347. E-mail address: [email protected] (K. Yamanouchi).
0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2004.06.009
identified in coincidence so that the charge number of the parent ions as well as its dissociation pathways can be specified definitively. Second, the three-dimensional (3D) momentum vectors of respective fragment ions are determined in the laboratory frame for every single event of the Coulomb explosion. Indeed, based on the correlation between the determined fragment momentum vectors, it became possible to identify the existence of sequential and non-sequential threebody explosion pathways [6] and to discuss the evolution of a nuclear wavepacket on the light-dressed potential energy surface (PES) [7] for CS2 in an intense laser field. In the present study, the two-body Coulomb explosion processes of acetonitrile (CH3 CN) [8] and deuterated acetonitrile (CD3 CN), CH3 CN2+ → CH3−n + + Hn CN+ (n = 0 − 2),
(1)
CD3 CN2+ → CH3−n + + Dn CN+ (n = 0 − 2),
(2)
in an ultrafast intense laser field (0.15 PW/cm2 , 70 fs) are investigated by the CMI method, where an ultrafast nuclear
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A. Hishikawa et al. / Journal of Electron Spectroscopy and Related Phenomena 141 (2004) 195–200
rearrangement process called hydrogen migration [9] occurring prior to the explosion process is identified. The fragment anisotropy becomes more isotropic as n increases from 0 to 2, showing that the rate of the Coulomb explosion becomes comparable with or even longer than the rotational period of the parent molecule as the migration of hydrogen atoms from the methyl group to the nitrile group proceeds. A quantitative analysis of the fragment anisotropy and the branching ratio of the three Coulomb explosion pathways (n = 0–2), competition between the hydrogen atom migration in the intense laser field and the two-body Coulomb explosion process is investigated.
2. Experimental The details of the experimental setup have been described previously [8]. Briefly, ultrashort laser pulses (70 fs, 0.15 mJ/pulse, 3 kHz) were focused at the crossing point of a sample molecular beam and the laser beam in an ultrahigh vacuum chamber with a base pressure of ∼3 × 10−11 Torr. The sample vapor of acetonitrile at room temperature was introduced into the chamber as a skimmed effusive molecular beam. The fragment ions produced through the Coulomb explosion of CH3 CN or CD3 CN at the laser focal spot were guided to a position sensitive detector (PSD) with delay-line readouts [10,11] by three parallel electrodes in the velocity mapping configuration [4,12]. The direction of the laser polarization vector was set to be parallel to the detector plane. The momentum vector pi = (pi x , pi y , pi z ) of the i-th fragment ion with mass mi and charge qi was determined in the x–y–z laboratory frame from the flight time ti and the position (xi , yi ) on the PSD as a pair of the fragment ions whose momentum components are given in terms of the mass mi and the charge qi of the respective ions by, pxi = αx mi xi /ti, y
ions in the interaction region were excluded based on the momentum conservation rule, pi = 0.
3. Results and discussion 3.1. Fragmentation of acetonitrile Fig. 1(a) shows a part of the TOF mass spectrum of the ion species produced from CH3 CN in the intense laser field (0.15 PW/cm2 ). The peaks observed at the mass numbers m/z = 12, 13, and 28 can be unambiguously attributed to C+ , CH+ and H2 CN+ . On the other hand, two alternative assignments are possible for the respective peaks at m/z = 14, 15, 26, and 27, depending on whether the C C bond or the C N bond is broken in the Coulomb explosion process. For example, the peak at m/z = 15 may be assigned either to NH+ formed through the C N bond breaking after the migration of a hydrogen atom, or to CH3 + produced by the C C bond breaking with no hydrogen atom migration. A secure assignment of the fragment ions can be achieved from the comparison of the peak intensity pattern of CH3 CN with that of deuterated acetonitrile (CD3 CN) shown in Fig. 1(b). For example, two strong peak at m/z = 14 and 15 in Fig. 1(a) should correspond, respectively to the peaks at m/z = 16 and 18 in Fig. 1(b), showing that the peaks at m/z = 14 and 15 are CH2 + and CH3 + rather than N+ and NH+ . Similarly, the prominent peaks at m/z = 26 and 27 are assigned to CN+ and HCN+ , instead of C2 H2 + and C2 H3 + , respectively, suggesting that the fragmentation of acetonitrile in the intense laser field is dominated by the C C bond breaking.
(3)
pi = αy mi yi /ti,
(4)
pzi = αz qi ti (Urep − Uacc )/d,
(5)
where xi and yi are the shifts from the reference position in the x–y coordinate system where an ion released with px = py = 0 hits the detector, and ti is the time delay with respect to the reference flight time when an ion with pz = 0 reaches the detector, and Urep and Uacc represent the electric potentials of the repeller and the accelerator electrodes, respectively. A set of the correction factors, (αx , αy , αz ), in Eqs. (3)–(5) is introduced to account for a weak inhomogeniety in the electrostatic field in the ion extraction region of the TOF mass spectrometer in the velocity mapping configuration. False coincidence events originating from more than two parent
Fig. 1. The TOF spectra of CH3 CN (a) and CD3 CN (b) in intense laser fields (70 fs, 0.15 PW/cm2 ). From the comparison of the peak intensity patterns, the peaks observed in the mass range are securely assigned to the fragment ions produced after the C-C bond breaking (see text). Other small peaks originate from residual gases in the vacuum chamber.
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3.2. Coincidence ion images Three different two-body Coulomb explosion pathways are identified as in Eq. (1), that is, the direct C C bond breaking without the migration of a hydrogen atom (n = 0) and the C C bond breaking after the single (n = 1) and double (n = 2) hydrogen migration. The coincidence ion images of the CH3−n + fragments produced through the three pathways are shown in Fig. 2(a)–(c). The n = 0 pathway exhibits a pair of clear crescent-like features, showing that the most of the fragment ions tend to be ejected in the laser polarization direction (ε). On the other hand, an anisotropy in the n = 1 pathway is less pronounced, and the nearly isotropic distribution is observed for the n = 2 pathway. This strong correlation between the fragment anisotropy and the extent of the hydrogen migration is also identified in the coincidence ion images in Figs. 2(d)–(f) for the two-body Coulomb explosion of the deuterated acetonitrile, CD3 CN2+ → CD3−n + + Dn CN+ (n = 0–2).
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At the momentum value p ∼ 95 × 103 amu m/s where the peak is located in the momentum distributions for all the observed two-body pathways, the fractions (f) of the false events are estimated to be f < 0.03 [8], which secures that the observed coincidence events originate from the two-body explosion of single parent ions. On the other hand, weak features commonly identified at p ∼ 0 in Figs. 2(a)–(c) are attributed mostly to the false events (f ∼ 1) [8]. These false events are attributed to the fragmentation through neutral pathways, e.g., CH3 CN+ → CH3−n + + Hn CN or CH3−n + Hn CN+ (n = 0–2), in which either one of the two fragments is neutral. The relative yields of the respective explosion pathways can be derived from the number of coincidence events. For CH3 CN, the relative yields for the n = 1 and 2 pathways with respect to the n = 0 pathway are 1.3(1) and 1.0(1), respectively, while for CD3 CN the yield ratios with respect to the n = 0 pathway is 0.9(1) and 0.7(1) for the n = 1 and 2 pathways, respectively. The comparable yields for the three explosion pathways show that the migration of hydrogen atoms compete with the two-body Coulomb explosion of acetonitrile in the intense laser fields (0.15 PW/cm2 ). 3.3. Fragment anisotropy in Coulomb explosion of CH3 CN2+ and CD3 CN2+
Fig. 2. The two-dimensional coincidence ion images of (a) CH3 + , (b) CH2 + , (c) CH+ produced through the two-body Coulomb explosion of CH3 CN2+ and those of (d) CD3 + , (e) CD2 + , (f) CD+ produced from CD3 CN2+ . The weak central features commonly observed in these maps originate from the false coincidences. The anisotropy appearing in the thin circular distribution with respect to the direction of the laser polarization vector (ε) tends to become less pronounced as n increases from 0 to 2 reflecting the lifetime lengthening of the doubly charged parent ions after the hydrogen migration from the methyl group to the nitrile group.
In the CMI measurements, the momentum distribution of the fragment ions are determined in the three-dimensional momentum space so that their angular distribution, I(θ), with respect to the laser polarization direction can be derived in a straightforward manner without a complicated mathematical procedure such as an inverse Abel transformation. The angular distribution can be derived from a thin slice of the three-dimensional momentum distribution at pz ∼ 0 as shown in Fig. 3 for the three explosion pathways (n = 0–2) of CH3 CN and CD3 CN. It can be seen in these figures that the fragment ions tend to be ejected more isotropically as the hydrogen migration from the terminal carbon to the nitrile group proceeds. The extent of the anisotropy in the explosion pathways can be evaluated quantitatively by the expectation value of the squared cosine,
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Fig. 3. The angular distributions of the fragment ions for the two-body Coulomb explosion of CH3 CN2+ through the (a) n = 0, (b) n = 1, (c) n = 2 pathways and those of CD3 CN2+ through the (d) n = 0, (e) n = 1, (f) n = 2 pathways derived from a thin slice of the momentum vector distribution at pz ∼ 0. The results of the least-squares fit with the analytical expression (Eq. (6)) incorporating the effect of molecular rotation are shown for comparison (solid lines). The fit was improved for the n = 2 pathways ((c) and (f)), when the effect of off-axis fragment ejection in the molecular frame was taken into account (dashed lines).
As the lifetime becomes longer, the angular distribution of the fragment ions tends to become more isotropic due to the overall rotational motion of the parent molecules. It should also be noted that the extent of the fragment anisotropy could be lowered depending on the direction of the fragment recoil in the molecular frame. Unlike diatomic molecules, the recoil direction of the fragment ions from a polyatomic molecule is not always along the molecular principal axis. Incorporating these three factors mentioned above, i.e., (i) the angular distribution of the molecular axis of the parent molecules in the space fixed frame, Dmol (θ), (ii) the extent of the overall rotation of the parent molecules within a finite 0 of the dissociation lifetime, and (iii) the ejection angle of θm fragment ions with respect to the molecular axis of the parent molecule (see Fig. 4), the angular distribution of a fragment ion, I(θ s , ϕs ), can be expressed using a product of Legendre polynomials, Is (θs , ϕs ) =
1 1 0 aL cL PL (cos θm )PL (cos θs ), (6) 4π 2L+1 L
where the coefficients aL ’s are given by the Legendre expansion of Dmol (θ), π 2 aL = 4π (2L + 1) Dmol (θ)PL (cos θ) sin θ dθ. (7) 0
The coefficients cL reflecting the effect of the molecular rotation are expressed as cL = 4π2 (2L + 1)
π 0
Drot (θr )PL (cos θr ) sin θr dθr ,
(8)
Fig. 4. The coordinate systems describing the ejection direction of the fragment ions. The laser polarization direction is parallel with the Z-axis of the space-fixed X–Y–Z coordinate system. The direction of the z-axis of the molecular fixed coordinate system at the instance of the laser irradiation is defined by the polar angle θ, and the z-axis is transferred into the z -axis of another molecular fixed coordinate system by the rotation by the polar angle θ r . The fragment ejection direction (θ s , ϕs ) in the space fixed coordinate 0 in the second molecular fixed system is represented by the polar angle θm frame.
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as a Legendre expansion of the probability Drot (θ r ) of finding a molecule dissociating after the rotation by an angle θ r in the molecular frame [13], Drot (θr ) =
1 4π2 τω
×
sin θr
exp(−(θr /τω)) + exp((θr − 2π)/τω) 1 − exp(−2π/τω)
, (9)
and are explicitly expressed as (L = 0) 1 L/2 cL = (1/τω)2 + (2k − 1)2 (L = 2, 4, . . .), (2L + 1) (1/τω)2 + (2k)2 k=1 (10) where τ and ω (=2π/τ rot ) represent, respectively the dissociation lifetime of a molecule and its rotational frequency. It is assumed that the explosion for the n = 0 pathway occurs immediately after the formation of the doubly charged acetonitrile (τω «1), and the fragments are ejected exclusively 0 = 0. This along the molecular axis, i.e., cL = 0 for L ≥ 2 and θm means that the observed sharp angular distribution for the n = 0 pathway represents Dmol (θ). Thus, the angular distribution of the fragments is expressed as Is (θs ) = Dmol (θs ) =
1 aL PL (cos θs ). 8π2
(11)
L
The results of the least-squares fit to Eq. (9) are shown in Fig. 3(a) and (d) for CH3 CN and CD3 CN, respectively, where the Legendre functions with L = 0, 2, 4, 6, 8 are used to achieve the convergence of the fit. With the five coefficients aL (L = 0, 2, 4, 6, 8) determined for the n = 0 pathway representing Dmol (θ), the least squares fit to Eq. (6) is performed for the n = 1 and 2 pathways by treating τω as a fitting parameter. The observed angular distributions for the n = 1 pathway for CH3 CN and CD3 CN are well reproduced when τω = 0.78(1) and 1.3(1), respectively, as shown in Fig. 3. Since the rotational periods, τ rot = 2B0 (
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0 ( =0) off-axis ejection of the fragments at a non-zero angle θm 0 are adopted in the molecular frame. When both τω and θm as variable parameters, the observed distributions are fitted 0 = 29(4)◦ for CH CN and τω > well with τω > 10 and θm 3 0 = 22(4)◦ for CD CN. From the lower limit values 10 and θm 3 for τω, τ > 3.8 ps and τ > 3.5 ps are obtained for CH3 CN 0 , sugand CD3 CN, respectively. The large offset angles, θm gest that substantial skeletal structural deformation proceeds for the n = 2 pathway associated with the hydrogen migration at the doubly charged stage. Since an off-axis ejection of fragments is also expected to occur for the n = 1 pathway, the dissociation lifetimes determined above for n = 1 may be regarded as the lower limit values. As described in Section 3.2, the three two-body pathways (n = 0–2) have comparable yields for both normal and deuterated species. The relative yields for the n = 1 and 2 pathways with respect to the n = 0 pathway, Φ1 = 1.3(1) and Φ2 = 1.0(1), respectively, may reflect the ratio of the number of the doubly charged parent molecules prepared in the three different states from which dissociation proceeds along the n = 0–2 pathways, respectively. Alternatively, the branching ratios reflect the competition between the hydrogen migration and the Coulomb explosion, occurring after the doubly charged parent molecules are prepared in the common state. In the latter case, the migration of the first hydrogen atom from the methyl group to the nitrile group is considered to proceed about twice as fast as the direct Coulomb explosion process in the n = 0 pathway to account for sum of the relative yields, Φ1 + Φ2 = 2.3, for the n = 1 and 2 explosion pathways occurring after the hydrogen migration. The branching ratio, Φ1 /Φ2 = 1.3, may suggest that the migration of the second hydrogen atom proceeds on a time scale comparable with or slightly shorter than the dissociation lifetime of the n = 1 pathway, τ ∼ 0.30 ps. For CD3 CN, the relative yields of the n = 1 and 2 pathways, Φ1 = 0.9(1) and Φ2 = 0.7(1), are smaller than the corresponding values of CH3 CN, indicating that the migration rate of the hydrogen atoms from the methyl group to the nitrile group for CD3 CN is slower than that for CH3 CN, indicating that the latter model in which the dissociation and the hydrogen migration compete is more plausible. The sum of the relative yields, Φ1 + Φ2 = 1.6, for CD3 CN indicates that the migration rate of the first hydrogen atom is 1.6/2.3 = 0.7 times slower for CD3 CN than for CH3 CN, if the dissociation lifetime for the n = 0 pathway is assumed to be unchanged by the deuteration of the three hydrogen atoms. The migration of hydrogen atoms from the methyl group to the nitrile group as well as the C–C–N skeletal deformation may also occur in the singly charged stage of acetonitrile prior to the formation of the doubly charged acetonitrile if these processes could proceed within the time scale of the laser pulse duration. The contribution from these processes in the singly charged stage to the fragment anisotropy could be investigated by a pump-and-probe scheme with a probe laser whose pulse duration is much shorter than the pump laser pulse.
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4. Summary The two-body Coulomb explosion of acetonitrile (CH3 CN) and deuterated acetonitrile (CD3 CN) in an intense laser field (0.15 PW/cm2 , 70 fs) was investigated by the coincidence momentum imaging method. Three different explosion pathways for the respective species, i.e., CH3 CN2+ → CH3−n + + Hn CN+ (n = 0–2) and CD3 CN2+ → CD3−n + + Dn CN+ (n = 0–2), were securely identified, indicating the existence of the hydrogen migration process occurring prior to the two-body explosion. From the comparable yields of the three explosion pathways (n = 0–2), it was inferred that the migration of hydrogen atoms proceeds in competition with the two-body Coulomb explosion. The angular distribution of the fragment ions with respect to the laser polarization direction becomes more isotropic as the number of the migrated hydrogen atoms n increases. The least-squares fits of the observed anisotropic fragment distribution by a model in which (i) the initial angular distribution of the molecular axis just after the photoabsorption, (ii) the effect of molecular rotation of the parent molecule with a finite lifetime, and (iii) the off-axis ejection of fragment ions in the molecular frame are incorporated and the dissociation lifetimes of the doubly charged acetonitrile as well as the timescale of the hydrogen migration are determined. It was also inferred that the doubly charged parent molecules undergo the substantial skeletal deformation in intense laser fields along the C–C–N bending coordinate prior to the two body explosion through the n = 2 pathway as in the case of CO2 [16]. Acknowledgements The authors thank Ms. Kyoko Doi for her assistance for the present study. The present work was supported by the
CREST fund (1997–2001) from the Japan Science and Technology Corporation, the research grant from Graduate School of the University of Tokyo, and the Grant-in-Aid for Priority Areas for Control of Molecules in Intense Laser Fields and the Grant-in-Aid for the 21st Century COE Program for Frontiers in Fundamental Chemistry from Ministry of Education, Culture, Sports, Science and Technology.
References [1] K. Yamanouchi, Science 295 (2002) 1659. [2] A. Hishikawa, A. Iwamae, K. Yamanouchi, Phys. Rev. Lett. 83 (1999) 1127. [3] A. Iwasaki, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 346 (2001) 379. [4] H. Hasegawa, A. Hishikawa, K. Yamanouchi, Chem. Phys. Lett. 349 (2001) 57. [5] C. Cornaggia, Phys. Rev. A 54 (1996) 2555. [6] A. Hishikawa, H. Hasegawa, K. Yamanouchi, Chem. Phys. Lett. 361 (2002) 245. [7] A. Hishikawa, H. Hasegawa, K. Yamanouchi, Chem. Phys. Lett. 388 (2004) 1. [8] A. Hishikawa, H. Hasegawa, K. Yamanouchi, Phys. Scr. T110 (2004) 108. [9] H.F. Schaefer III, Acc. Chem. Res. 12 (1979) 288. [10] J. Ullrich, R. Moshammer, A. Dorn, R. D¨orner, L.P.H. Schmidt, H. Schmidt-B¨ocking, Rep. Prog. Phys. 66 (2003) 1463. [11] R. D¨orner, V. Mergel, O. Jagutzki, L. Spielberger, J. Ullrich, R. Moshammer, H. Schmidt-B¨ocking, Phys. Rep. 330 (2000) 95. [12] A.T.J.B. Eppink, D.H. Parker, Rev. Sci. Instrum. 68 (1997) 3477. [13] C. Jonah, J. Chem. Phys. 55 (1971) 1915. [14] M. Koivusaari, V.-M. Horneman, R. Anttila, J. Mol. Spec. 152 (1992) 377. [15] S. Alanko, M. Koivusaari, R. Paso, R. Anttila, J. Mol. Spec. 207 (2001) 89. [16] Y. Sato, H. Kono, S. Koseki, Y. Fujimura, J. Am. Chem. Soc. 125 (2003) 8019.