Femtosecond Time-Resolved Photofragment Rotational Angular

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Letter

Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics Maria E. Corrales, Peter S. Shternin, Luis Rubio-Lago, Rebeca de Nalda, Oleg S. Vasyutinskii, and Luis Banares J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b01874 • Publication Date (Web): 24 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry Letters

Femtosecond Time-Resolved Photofragment Rotational Angular Momentum Alignment in Electronic Predissociation Dynamics M. E. Corrales,† P. S. Shternin,‡ L. Rubio-Lago,† R. de Nalda,¶ O. S. Vasyutinskii,∗,‡ and L. Bañares∗,† †Departamento de Química Física I (Unidad Asociada I+D+i al CSIC), Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain ‡Ioffe Institute, Polytechnicheskaya 26, 194021 St. Petersburg, Russia ¶Instituto de Química Física Rocasolano, CSIC, C/ Serrano, 119, 28006 Madrid, Spain E-mail: [email protected]; [email protected]

1 ACS Paragon Plus Environment


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Abstract

This Letter presents an experimental and theoretical study of femtosecond timeresolved vector correlations in methyl iodide (CH3 I) electronic predissociation via the second absorption B-band at 201.2 nm. The time-evolution of the phenomenological anisotropy parameters βl was determined from time-resolved photofragment angular distributions obtained by means of the femtosecond laser pumpprobe technique coupled with velocity map imaging detection of vibrational ground state CH3 (ν=0) fragments and spin-orbit excited I∗ (2 P1/2 ) atoms. Theoretical interpretation of the experimental results was performed on the basis of a fitting procedure using quasiclassical theory, which elucidates vector correlations in photodissociation of symmetric top molecules. The results of the fitting are in very good agreement with the experimental data and demonstrate the important role of molecular excited state lifetimes, parent molecule and methyl fragment rotations, and methyl fragment angular momentum alignment, on the time-dependent electronic predissociation dynamics.

Graphical TOC Entry

J Vrecoil

εphot


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As was realized many years ago, angular momentum correlations play a significant role in molecular photodissociation dynamics. 1–5 Molecular interactions always occur through anisotropic forces and give rise to anisotropic angular momentum distributions, which may have relation to factors such as approach direction, scattering direction, or photon polarization, among others. Nowadays, the full recoil velocity distribution of state resolved photofragments can be detected in a highly multiplexed approach mainly based on the ion imaging 6 and velocity map imaging 7 techniques. The power of these techniques lies in their ability to reveal the molecular-frame dynamics and, in particular, coherent excitation mechanisms involving multiple electronic states and nonadiabatic processes at curve crossings and at the long-range. Investigations of the polarization of photofragment angular momenta have been carried over the years as a powerful tool to obtain unique information on the properties of the close coupling region of molecular interactions (see, for instance, Refs. 5,8–13 and references therein). However, most of the experimental studies dealt so far with the nanosecond timedomain, providing only information on the asymptotic (t → ∞) vector correlations and asymptotic values of the orientation and alignment parameters. Only a few papers have been published till now 12,14 where the role of the angular momentum polarization in the time-resolved photolysis was addressed. The extension of the vector correlation studies to the femtosecond time-domain has an undeniable advantage as it allows to use the full power of vector correlation methods 5,11 for detailed investigation of the real-time molecular dynamics. In this Letter we report a study of femtosecond time-resolved vector correlations in the predissociation dynamics of methyl iodide (CH3 I) via B-band absorption. A set of anisotropy parameters has been determined from time-resolved photofragment angular distributions using the pump-probe technique coupled with velocity-map imaging detection of the dissociation products CH3 (ν=0) and I∗ (2 P1/2 ) and then interpreted by means of a quasiclassical theory. Photodissociation of methyl iodide is known as a benchmark system for investigation



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of important properties of molecular dynamics. Considerable efforts have been devoted to study the dynamics of the photodissociation of CH3 I via the second absorption B-band and to determine the product branching ratios, 12,15,16 energy partitioning among the different degrees of freedom, lifetimes, 12,15 and stereodynamics. 13,17 All these works provided pieces of the main picture, while many questions still remain open. In this Letter, we demonstrate the important role of photofragment angular momentum alignment on the time-dependent electronic predissociation dynamics. The experimental procedure was similar to that described in detail elsewhere. 12 In brief, a beam of 3.5 mJ, 50 fs pulses at about 804 nm generated by a 1 kHz Ti:Sa laser system was split into two arms. One of those was used to produce the radiation either at 304.5 nm for (2+1) resonance enhanced multiphoton ionization (REMPI) probing of the I∗ (2 P1/2 ) atoms, or at 333.5 nm for (2+1) REMPI probing of the vibrational ground state CH3 (ν=0) fragments, in an optical parametric amplifier. Another arm provided radiation resonant with the 000 B-band vibronic transition in CH3 I at 201.2 nm by quadrupling the original radiation. The pump and probe beams were polarized horizontally, propagated collinearly and focused to the center of the interaction region with a molecular beam. Methyl iodide was seeded in He and expanded into vacuum through a piezoelectric pulsed valve. Ions formed in the interaction region were extracted perpendicularly by a set of open-lens electrodes that constituted an ion lens system operating in the velocity mapping configuration. 7 Figure 1 presents a sequence of I∗ (2 P1/2 ) Abel-inverted velocity map images obtained at several time delays td between the pump (at 201.2 nm) and probe (at 304.5 nm) laser pulses. The contribution in the center of the images in Figs. 1 and 3 were assigned to multiphoton disociative ionization processes which are out of the scope of the present study. CH3 I photodissociation via 000 B-band absorption is represented in each image shown in Fig. 1 by an outer anisotropic ring with intensity increasing with td , until it stabilizes at td >6 ps. The ring is associated with the CH3 (ν)+I∗ (2 P1/2 ) photodissociation channel, where several C–H stretch (ν1 ) and umbrella (ν2 ) methyl fragment vibrational modes are known to be


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200 fs

500 fs

1 ps

2 ps

3 ps

10 ps

Figure 1: Sequence of Abel-inverted iodine images in false color, as a function of the time delay td between the pump and probe laser pulses at 201.2 and 304.5 nm, respectively. The outer ring corresponds to formation of I∗ (2 P1/2 ) atoms in correlation with CH3 (ν) fragments in ν1 and ν2 vibrational states.

excited. 15 At short time delays, the ring intensity shows a strong perpendicular distribution and for larger delay times the reduction in the anisotropy is clearly observed. More quantitative information can be extracted from the photofragment angular distribution obtained by radial integration of the ring. For a one-photon dissociation and (n+1) REMPI detection scheme, the photofragment angular distribution can be expressed by an expansion over Legendre polynomials Pl (cos θ): 4 2n+2 σ X I(θ) = βl Pl (cos θ), 4π l=0


(1)


where θ is the angle between the recoil direction and the photolysis laser polarization, σ is the absorption cross section, and βl are anisotropy parameters. As the probe laser pulse is linearly polarized, the index l can take only even values. 5 For the I(2 P1/2 ) photofragment with total angular momentum J=1/2, no alignment can exist, the sum in Eq. (1) is truncated at l=2, and β2 coincides with the known anisotropy parameter β. 18

-0.2

2

-0.4

-0.6 Experimental Theory predissociation

-0.8

Theory direct dissociation

-1.0

Intensity / arb. units

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0

2

T

4

6

8

10

12

ime delay / ps

Figure 2: Experimental (open circles) and theoretical fit (colored lines) results for the I∗ (2 P1/2 ) product. Top panel: β2 parameter as a function of the time delay between the pump and probe pulses. Bottom panel: intensity transient which results from the angular integration of the main ring in the images as a function of time delay. The red and blue lines in both panels reflect the contribution from predissociation and direct dissociation mechanisms, respectively. See the text for details.

The anisotropy parameter β2 determined from the fitting of the angular distribution as a function of td is represented by open circles in the upper panel of Fig. 2. Open circles in the 6 ACS Paragon Plus Environment

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lower panel of Fig. 2 represents the I∗ (2 P1/2 ) photofragment intensity transient. As shown in Fig. 2, at early times, β2 ≈–1, in agreement with the known perpendicular nature of the 3

˜ 1 A1 transition. For longer time delays, the initial β2 diminishes to final values R1 (E) ← X

of about β2 ≈–0.5. Figure 3 depicts the CH3 fragment Abel-inverted velocity map images obtained as a function of td with probe laser pulses centered at 333.5 nm. Two distinct rings in the images with different extent of intensity and anisotropy reflect the formation of CH3 (ν=0) and CH3 (ν1 =1) in the I∗ (2 P1/2 ) channel. 12,15 As can be seen in Fig. 3, at short td , the intensity

200fs

500fs

x4

x2

2 ps

1 ps

3 ps

10 ps

Figure 3: Sequence of Abel-inverted methyl images in false color, as a function of the time delay between the pump and probe pulses at 201.2 and 333.5 nm, respectively. The observed rings correspond to formation of CH3 (ν=0) (outer ring) and CH3 (ν1 =1) (inner ring) products in correlation with I∗ (2 P1/2 ) atoms.

distribution of the CH3 (ν=0) fragments is coherent with a perpendicular transition, but is not zero at the poles of the ring. As td increases, the anisotropy decreases, and a nearly-



isotropic distribution is reached at the asymptotic limit of about 2 ps. The rotational angular momentum of the CH3 fragment can be aligned with respect to the transition dipole moment µ and the recoil velocity v. The angular distributions obtained by radial integration of the outer ring in Fig. 3 corresponding to CH3 (ν=0) fragments were fitted according to Eq. (1) with l ≤ 6. Open circles in Fig. 4 represent the time evolution of the β2 , β4 , and β6 anisotropy parameters and the CH3 (ν=0) intensity transient.

6

0.08

0.00

-0.08

4

0.0 -0.1 -0.2 0.0 -0.3

2

-0.6 -0.9 Intensity / arb. u.

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Experimental Theory predissociation Theory direct dissociation

0

2

4

6

8

10

12

14

16

18

T

ime delay / ps

Figure 4: Experimental (open circles) and theoretical fit (colored lines) results for the CH3 (ν=0) product. From top to bottom: β6 , β4 , and β2 anisotropy parameters obtained as a function of time delay between the pump and probe pulses and the intensity transient. The red and blue lines in all panels reflect the contribution from predissociation and direct dissociation mechanisms, respectively.

Interpretation of the experimental results was performed on the basis of the quantum me8 ACS Paragon Plus Environment

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chanical theory on vector correlations in photodissociation beyond the axial recoil limit, 19–23 which was modified to the time-dependent form within the quasiclassical approximation. Under the experimental conditions used, the parameters βl in Eq. (1) can be presented in the form: 23 βl =

X K,kd ,qk

(−1)qk l0 √ Ckl d00K0 CKq PK cK kd qk , k kd −qk 2K + 1

(2)

where kd = 0, 2 and K = 0, 2, 4 are ranks related to the photolysis light polarization and the photofragment angular momentum alignment, respectively, qk is a component of the l0 are Clebsch-Gordan coefficients, and PK are ranks K and kd onto the recoil axis, CKq k kd −qk

linestrength factors. 5 21,22 which contain The terms cK kd qk in Eq. (2) are the anisotropy transforming coefficients,

complete information on the dissociation dynamics and can (at least in principle) be directly determined from the quantum scattering theory. Depending on the quantum numbers K, kd , and qk , the coefficients cK kd qk reflect different features of the photodissociation mechanism: √ transition symmetry, coherent/incoherent excitation, and others. 5 In particular, c000 = − 3 q 3 β, while all other coefficients cK and c020 = kd qk are associated with the photofragment 2 angular momentum polarization. According to Ref. 23, in direct photodissociation of a prolate symmetric top molecule in the high-J limit, the coefficients cK kd qk can be presented in the form: kd cK kd qk ∝ 2 cos(γs qk )dqk qk (γpr )

(3)

where dkqkdqk (γpr ) is the Wigner d-function, while γpr and γs are the classical precession and self-rotation angles, 23 respectively, during dissociation. Equation (3) is valid only when the lowest rotational K-state of the parent molecule is populated, that fits the CH3 I case in typical molecular beam conditions. In this work we introduce time-dependent coefficients cK kd qk (t) by generalizing a simple classical model by Jonah 24 and Busch and Wilson 25 . Assuming that the photodissociation 9 ACS Paragon Plus Environment


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occurs for a time t, the precession and self-rotation angles can be represented as γpr (t) = ωJ t+ γ0 , and γs (t) = ωs t, respectively, where J is the parent molecule total angular momentum. The permanent angular velocities ωJ and ωs are given by the known expressions ωJ ≈ 4πBJ and ωs ≈ 4π(A − B), where A and B are the CH3 I rotational constants. The angle γ0 , which will be called later scattering angle, can be mostly associated with the H3 –C–I axis bending during photodissociation. 26,27 The time-dependent anisotropy transforming coefficients cK kd qk (t) can be defined by averaging Eq. (3) over the J-states and over the Poisson dissociation probability, W (t) = e−t/τ /τ :

cK kd qk (t)

X BJ(J+1) 2cK − kd qk (0) (2J + 1)e kB T = N J

Zt

cos[4π(A − B)qk t0 ]dkqkdqk (4πBJt0 + γ0 )W (t0 ) dt0 ,

0

(4) where T is the rotational temperature of the parent molecule in the molecular beam, τ is an average excited state lifetime, kB is the Boltzmann constant, and N is the rotation normalization constant. The coefficients cK kd qk (0) in Eq. (4) describe the initial angular momentum polarization in the excited parent molecule at t = 0. After substituting Eq. (4) into Eqs. (2) and (1), a global fit of the data presented in Figs. 2 and 4 was performed using cK kd qk (0), γ0 , τ , and T as fitting parameters and considering CH3 I photodissociation via the predissociative 3 R1 and direct dissociative 3 A1 (E) states. 13,17 The rotational temperature T was considered as a global fit parameter, while the lifetime τ and the cK kd qk (0) coefficients with K = 0, 2 were assumed to be the same for both photofragments, but different for each photodissociation channel. The scattering angle γ0 was assumed to be different for the I∗ and CH3 fragments and labelled by superscript indices i and m, respectively. For the direct dissociation channel, these angles were set to zero. A ˜ 1 A1 was explicitly included perpendicular character of the optical transition 3 R1 (E) ← X into the fitting procedure. The theoretical time-dependent signal was convolved with the instrument response function (IRF), which was determined in a separate experiment and 10 ACS Paragon Plus Environment

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assumed to be a Gaussian with a full-width-half-maximum (FWHM) of δcc = 420 fs. The temperature T , the ratio between the two dissociation mechanisms, a, the lifetimes of the predissociative and direct dissociative excited states, τp and τd , and the scattering angles are given in Table 1. The initial values of the anisotropy transforming coeffiecients cK kd qk (0) are not shown due to lack of space. Table 1 Parameter τd (fs) τp (fs) T (K) γ0i (rad) γ0m (rad) a

fit value 284(6) 1485 (7) 24.8 (0.4) 0 0.53(0.01) ≤ 0.002

The mean dissociation lifetimes τd and τp stand for direct dissociation and predissociation mechanisms, respectively and a is the branching ratio between them. T is the rotational temperature of the parent molecule in the molecular beam and γ0i and γ0m are the scattering angles for the iodine and methyl fragments, respectively (see the text for more details). Numbers in parentheses are fit uncertainties on the 90% confidence level.

As can be seen in Table 1, direct dissociation and predissociation mechanisms are characterized by significantly different lifetimes τd and τp . In the conditions of the present femtosecond experiments characterized by a large pump pulse linewidth of about 1 nm, contribution from the predissociation mechanism dominates over the direct photodissociation, with a ratio of a ≤0.002. This conclusion is in qualitative agreement with the nanosecond photodissociation CH3 I experiments 13 where the area under the relatively narrow predissociation peak dominates over the area under the much less intense but more extended direct dissociation spectral band. The resulting intensity transients are shown by solid curves in the lower panels in Figs. 2 and 4, in comparison with the experimental data (open circles). The best-fit theoretical temporal dependencies of the β2 parameter for the I∗ (2 P1/2 ) photofragment are depicted by solid curves in the top panel of Fig. 2. As is clear from



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the figure, the experimental β2 values are well reproduced by the predissociation mechanism alone. The direct process (blue line) is characterized by a β2 =–1 value near td = 0, in agreement with fast dissociation via a perpendicular optical transition, and by an asymptotic value of β2 =–0.9 due to molecular axis rotation on the angle ωJ τd ' 15◦ , in agreement with the results of nanosecond experiments. 13 The predissociation mechanism (red line) is characterized by β2 ≈ −1 near t = 0 and by a somewhat more isotropic asymptotic values, β2 ≈ −0.5. This result can be interpreted by the perpendicular character of the optical transition to the 3 R1 excited state and a relatively large value of the molecular axis rotation angle ωJ τp ' 90◦ for the predissociation channel containing a large area of attractive interfragment forces. The results of the fitting for the time-dependent β2 , β4 , and β6 parameters for the CH3 fragment are presented in Fig. 4 along with the intensity transient. Again, the experimental results are well reproduced by considering the predissociation mechanism alone with a negligible contribution from the direct dissociation. Small discrepancies from the model near td = 0 in Figs. 2 and 4 can be attributed to several reasons: (i) the approximation used for IRF can result in fit deviations near td = 0; ˜ 1 A1 (ii) the model does not take into account small parallel contributions to the 3 R1 (E) ← X transition considered by theory; 28 (iii) the overlapping of the pump and probe laser pulses near td = 0, where the field-free model used can be inconsistent. As can be seen in Eq. (2), the theoretical expressions for the parameters β0 , β2 , β4 , and β6 contain contributions from several anisotropy transforming coefficients cK kd qk (t). Therefore, each CH3 fragment parameter βl depends in general on the angular momentum alignment. This important result should be taken into consideration when comparing the parameters βl obtained for iodine and methyl fragments. In particular, the parameter β2 for CH3 in Fig. 4 can be expressed as a linear combination of the coefficients c020 , c200 , c220 , c222 , c420 , and c422 . As a result, the time-dependent β2 parameter for CH3 (ν=0) in Fig. 4 differs from that for iodine in Fig. 2 due to the influence of the CH3 (ν=0) rotational angular momentum alignment.


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Another important feature of the CH3 fragment experimental data is that the satisfactory theoretical fit reveals a considerably large scattering angle value γ0m (see Table 1). In the case of CH3 I photodissociation, this angle can be mostly associated with H3 –C–I bending during photodissociation. As it is known, 26,27 the bending vibrational motion in CH3 I photodissociation plays an important role in breaking the symmetric top symmetry, originating the non-adiabatic coupling in the CH3 I excited states. Moreover, the bending motion leads likely to excitation of umbrella (ν2 ) and C–H symmetric stretch (ν1 ) modes in the methyl fragment 15,26 and to rotation of the methyl group preferentially perpendicular to the top axis. 26,29 The latter effect contributes significantly to the observed results, giving rise to “coherent” anisotropy transforming coefficients cK kd qk (t) with qk 6= 0, which are responsible for the Ω, Ω0 coherence, where Ω is the component of the fragment angular momentum along the recoil axis. The existence of these coefficients in Eq. (2) significantly improves the quality of the fit. A detailed theory of this effect will be published later. The experimental procedure and the theoretical machinery reported in this paper can be used for investigation of photodissociation dynamics of any symmetric top molecule. As it has been shown, the femtosecond time-resolved approach provides a direct insight on the role of the angular momentum polarization, internal dissociation dynamics, parent molecule rotation, and molecular fragment rotation. While the reduction of residual anisotropy can be deduced from nanosecond experiments, information on the fragments polarization and dissociation dynamics as a function of time can be obtained only by means of the realtime approach. Moreover, as shown in this Letter, the used theoretical approach allows for inferring all relevant anisotropy transforming coefficients and scattering angles even for a single REMPI scheme. In summary, by means of the femtosecond pump-probe laser technique in combination with velocity map imaging, the time-dependent angular distributions of the CH3 (ν=0) and I∗ (2 P1/2 ) fragments from CH3 I photodissociation via the second absorption B-band have been recorded and parameterised in terms of the anisotropy parameters. The results obtained have



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been interpreted by a quasiclassical theoretical model describing a symmetric top molecule photodissociation in both direct dissociation and electronic predissociation channels. Contribution from electronic predissociation has been found to be the dominant photodissociation mechanism. Comparison between the time-resolved experimental results and the theoretical model manifests the influence of the rotational angular momentum alignment, bending motion, Ω, Ω0 coherent effects, and methyl fragment rotation on the photodissociation dynamics.

Acknowledgement The work of the St. Petersburg group has been supported by the Russian Science Foundation under project No. 14-13-00266. The work of the Madrid group has been financed by the Spanish Ministry of Economy and Competitiveness (MINECO) through grants CTQ201237404-C02-01 and CTQ2015-65033-P. This research was performed within the Unidad Asociada Química Física Molecular between Departamento de Química Física of Universidad Complutense de Madrid (UCM) and Consejo Superior de Investigaciones Científicas (CSIC). The facilities provided by the Center for Ultrafast Lasers at UCM are gratefully acknowledged.

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Femtosecond Time-Resolved Photofragment Rotational Angular

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