Article pubs.acs.org/JPCA
Ultrafast Relaxation Dynamics of Uracil Probed via Strong Field Dissociative Ionization Spiridoula Matsika,*,† Michael Spanner,‡ Marija Kotur,§ and Thomas C. Weinacht*,∥ †
Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States National Research Council of Canada, Montreal, QC, Canada § Department of Physics, Lund University, 221 00 Lund, Sweden ∥ Department of Physics and Astronomy, Stony Brook University, Stony Brook, New York 11790, United States ‡
S Supporting Information *
ABSTRACT: We study the ultrafast relaxation dynamics of uracil excited to the first bright ππ* state (S2) by an ultrafast laser pulse in the deep ultraviolet (central wavelength λ0 = 260 nm). With a unique combination of strong field dissociative ionization measurements, state of the art strong field ionization calculations, and high level ab initio calculations of excited neutral and ionic states at critical points along the neutral potentials, we are able to gain a detailed picture of the relaxation dynamics of the molecule, which resolves earlier disagreements regarding measurements and calculations of the relaxation.
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INTRODUCTION
the excited state, explores multiple relaxation pathways simultaneously.8 The mechanism of how the decay occurs in uracil has been discussed extensively in the literature, relying primarily on theoretical studies.9−25 The quantum chemical calculations show that initial absorption leads to the S2 state, which is a bright ππ* state. The S1 state is a dark nπ* state a few tenths of an eV below S2. There are conical intersections (CIs) between S1 and S2, as well as between S1 and the ground state that can lead to radiationless relaxation to the ground state. Three general types of CIs between S1 and S0 have been found: an ethylenic one involving a twist around the double CC bond (where the ππ* state crosses with the closed shell ground state), one that involves CO stretching and out of plane deformation (where the nπ* state crosses with the ground state), and one that involves ring-opening. The main CI contributing to the relaxation, however, is the ethylenic one. Three-state CIs between S2−S1 and S0 have also been found, although they are also not energetically favorable for radiationless decay.26 The pathways that have been proposed for the decay are as follows. After UV absorption, there can be direct decay from the ππ* excited state to the ground state through a barrierless path involving two conical intersection seams: initially the S2/S1 CI and then S1/S0. Alternatively, population can be trapped on
The ultrafast relaxation of DNA and RNA bases is both of fundamental importance for our understanding of molecular dynamics in polyatomic molecules and of great practical interest in understanding the photoprotection of DNA and RNA.1,2 Uracil is a base found in RNA, which differs from thymine (a base in DNA) by the lack of a methyl group. During the past decade, measurements and calculations on the relaxation of uracil have shed significant light on the excited state dynamics following the absorption of UV light, but there are some points that are not clear, and there is disagreement in the literature on the details. Several pump−probe techniques have been applied to uracil in the gas phase. Initially, a pump−probe study that used a 267 nm pulse to excite the molecules and a strong field nearinfrared laser to ionize indicated that the ultrafast decay occurs in 2.4 ps.3 Another study using a 267 nm excitation pulse and ionization as a probe found two lifetimes: 0.13 and 1.05 ps.4 Time-resolved photoelectron spectroscopy has also been used to study the excited population decay using pulses at 250, 267, and 277 nm to excite the molecules and found the decay to be governed by three time constants: one less than 0.05 ps, a 0.53 ps, and a 2.4 ps.5 A dark state that has a lifetime of nanoseconds has also been observed6,7 and has been assigned to trapping on the lower nπ* state. In our previous work using strong field dissociative ionization as a probe, we found decay times of 0.07−0.09 ps and 2−3 ps, and like several other DNA/RNA bases, evidence of nonlocal relaxation, i.e., the wavepacket on © 2013 American Chemical Society
Received: August 12, 2013 Revised: November 4, 2013 Published: November 20, 2013 12796
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the S2 state for a couple of picoseconds, giving rise to the observed lifetime of 2.4 ps, followed by decay either to the ground state or to the dark S1 state. After the S2/S1 CI, some population will also be trapped on the dark S1 minimum and remain there probably for nanoseconds. The involvement of triplet states has also been proposed for uracil in solution,27,28 although theoretical calculations in the gas phase suggest that they do not contribute to the fast decay times in the gas phase.20 Figure 1 illustrates in a simple 1D cartoon the main
Article
COMPUTATIONAL METHODS
The ionization yields were computed using the time-dependent resolution-in-ionic-states method (TD-RIS), outlined in ref 29. The TD-RIS method mixes orbital-based multielectron wave functions, used to represent the bound states of the neutral molecule and cation species, with Cartesian grids used to support the liberated electron wavepackets following ionization. This relatively new computational method has been successfully employed to interpret strong field experiments on molecules such as CO,30,31 NO2,32 butane and butadiene,33,34 and uracil.35,36 The full details of the ionization computations as implemented for uracil can be found in ref 36. The geometries of uracil at various points along the PESs are taken from previous publications9,15 where the Multi-Reference Configuration Interaction (MRCI) approach was used for the optimizations. The coordinates are given in the Supporting Information. The neutral and ionic wave functions used as inputs to the strong-field ionization calculations were obtained using state-averaged complete active space self-consistent field (SA-CASSCF) calculations and the Dunning aug-cc-pVDZ basis set. The active space for the neutral calculations included 14 electrons in 10 orbitals (including the 8 π orbitals and 2 nO lone pairs), and 4 states were averaged in the CASSCF, while the ionic calculations used an active space of 13 electrons in 10 orbitals and averaged over 8 ionic states. An active space of m electrons in n orbitals is denoted here as (m,n). The energies of the neutral and ionic states at these previously optimized geometries were recalculated here including correlation at the CCSD level with the 6-311+G(d) basis set. EOM-EE-CCSD was used for the excited states, while EOM-IP-CCSD was used for the ionic states.37 The energy of the barrier on the S2 PES was calculated and compared to previous studies. The geometries of the S2 minimum and the S2 transition state were obtained using MRCI with the cc-pVDZ basis set and a (12,9) active space (resulting in 607,320 configuration state functions (CSFs)). All π orbitals and only one lone pair were included in the CAS since it has been found that the second nO orbital does not contribute to the low lying excited states.9 Single excitations from the CAS were included in the MRCI expansion. This is the same expansion used in previous work.9 Single-point calculations using these geometries were also performed using EOM-CCSD/6-311+G(d) and MRCI(12,9)/6-311+G(d) with single excitations from both the CAS and the σ orbitals (15,590,862 CSFs). Orbitals for the MRCI calculations were always obtained from a CASSCF calculation with the same active space as the MRCI expansion and three stated averaged (SA-3-CASSCF).
Figure 1. Cartoon showing the excited state dynamics in uracil after UV absorption.
features of the singlet potential energy surfaces (PESs) based on the experiments and calculations discussed above. The most contentious point is whether population can be trapped on the S2 surface, and this is related to how high the barrier on the S2 surface is, so that it can stop the population before reaching the S2/S1 CI. Many quantum chemical studies predict that there is either no barrier on the S2 surface or a very small one,9,16,25 and some dynamical studies have also shown no trapping on the S2 surface.18 However, other dynamical studies indicate that there can be trapping on the S2 surface.14,24 Accurate dynamical studies on the excited states of these polyatomic molecules are very challenging, and one has to compromise either on the level of electronic structure theory and/or the level of how the nuclear dynamics is carried out. Thymine, a methylated uracil, shows very similar characteristics and PESs, although the dynamics can have differences if motion of the methyl group is involved since the kinetics of a methyl group vs hydrogen are expected to be different.14,23 As it is difficult to obtain a definitive answer based solely on theory, it would be very valuable to have experimental evidence to help distinguish between trapping on the S1 or S2 surfaces. Here, we interpret experimental measurements with the aid of ab initio electronic structure and strong field ionization calculations. The combination of these techniques yields insight into the details of the relaxation dynamics of uracil.8,14 Our approach makes use of an ultrafast laser pulse in the deep UV (at 260 nm) to pump the molecules to the first bright excited state (S2). The pump pulse is followed by a time delayed intense near-infrared probe pulse, which ionizes the molecules to multiple states of the molecular cation, producing many fragments in a time-of-flight mass spectrometer. The electronic structure and strong field ionization calculations allow us to associate different molecular fragment ions with different relaxation pathways and determine the extent to which population remains on the S2 surface. Any involvement of the triplet states in the dynamics will not be addressed in this work, and it is not expected to affect our conclusions.
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EXPERIMENTAL METHODS Our measurements make use of an amplified ultrafast titanium sapphire laser system, which produces 30 fs laser pulses with a central wavelength of 780 nm and an energy of 1 mJ at a repetition rate of 1 kHz. These laser pulses are directed into a Mach−Zehnder interferometer, which is used for our pump− probe measurements. One arm of the interferometer has a third harmonic generation apparatus, which we use to generate pump pulses at a wavelength of 260 nm. The other arm of our interferometer is used to generate the probe pulses (at 780 nm) and contains a translation stage under computer control. The pump and probe pulses are combined on a dichroic beam splitter and focused using a fused silica lens into an effusive 12797
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PESs. These geometries are the minimum on the S2 PES, the minimum on the S1 PES, the minimum on the S0 surface (Franck−Condon (FC) point), and the conical intersection between S1 and S2 (S1/S2 CI). Ionization from the different parts of the PESs are qualitatively similar to ionization from the FC region, but there are differences as well. Ionization from S2 is always about an order of magnitude higher than ionization from S1. Ionization from S1 always leads to mostly D1, while ionization from S2 leads to a distribution of ionic states. Ionization from S1 to D1 can be explained by simple Koopmans’ Theorem arguments where removing an electron from the π* orbital starting from an nπ* state (S1) will lead to an ionic state with a single electron in the n orbital (D1). However, as we have discussed in previous publications, the distribution to other ionic states is different from what simple theories would predict.35,39 There are some small changes in the distributions for different location because the character of states changes somewhat and also the effective IPs change along the PES. It should be noted that the method used for ionization is not the same as the method used for optimizing the CI, so the effects of mixing of S1 and S2 at the CI are not obvious here. Table 1 shows the excitation and ionization energies for uracil at three different geometries, the ground state minimum,
molecular beam inside our time-of-flight mass spectrometer. In order to compensate for the difference in the focal length of the lens at the two frequencies, the probe pulses are sent through a pair of lenses to prepare a slightly divergent beam, which can then be focused to the same position as the pump pulses. The peak intensities of the pulses were about 0.3 TW/cm2 for the UV and about 10 TW/cm2 for the IR. We prepare an effusive beam of uracil by gently heating a powder sample. Care was taken to work at the lowest temperature that provides a useful molecular density at the focus of the lasers, and the lasers were focused near the nozzle of our molecular beam in order to achieve a relatively high number of molecules in the focal volume. The experimental setup is described in more detail in ref 38.
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RESULTS AND DISCUSSION The experimental outcome of our approach is fragment ion yields as a function of time. An essential step in interpreting our measurement results is to connect the time-dependent yields of the different molecular fragment ions with motion on the relevant neutral potential energy surfaces. We accomplish this with a combination of ab initio electronic structure calculations and strong field molecular ionization calculations. The strong field ionization (SFI) calculations tell us which cationic states are populated via SFI from which neutral states, and the ab initio calculations tell us which fragments are formed from the different cationic states. Figure 2 shows the ionization yields when ionization starts either on the S1 or S2 state at various important geometries, which represent stationary points or CIs along the neutral
Table 1. Excitation and Ionization Energies of Uracil in eV at Three Geometries, the S0 Minimum, the S1 Minimum, and the S2 Minimum, Calculated Using EOM-EE-CCSD and EOM-IP-CCSD with the 6-311+G(d) Basis Set; All Energies Are Given Relative to the Energy of the Ground State at S0 Minimum; the Symmetry Labels Only Apply to the Energies at the S0 Minimum Geometry Where the Molecule Is Planar S0 (A′) S1 (A″) S2 (A′) D0 (A″) D1 (A′) D2 (A″) D3(A′) D4 (A″)
S0 min
S2 min
S1 min
0 5.31 5.75 9.41 10.11 10.50 11.08 12.94
1.31 5.00 5.54 10.42 10.68 11.43 12.45 14.27
1.10 4.71 5.64 10.32 10.48 11.31 12.18 14.07
the S1 minimum, and the S2 minimum. The experimental vertical ionization potential (VIP) of uracil from photoelectron spectroscopy measurements is 9.45−9.60 eV,40,41 in very good agreement with our values. Although the IP is reproduced very well with the calculations, the excitation energies, especially the one to the S2 state, are very difficult to obtain accurately and one has to go to increased basis sets and include triple excitations, as has been shown previously.42 This means that the IPs from the excited states found here will be smaller than the true values, but we still believe the relative values and the trends are valid. The energies of the ionic states are qualitatively similar to the ones we published in previous work using multiconfigurational perturbation theory techniques.43 The effective IP for ionization from S2 to D0 in the FC region is 3.7 eV, while when the wavepacket has moved to the S2 minimum it is 4.9 eV. When the wavepacket has reached the S1 minimum and ionization occurs from the S1 state, the IP is 5.6 eV. It is evident from these values that the effective IP increases dramatically away from the FC region along the excited state surfaces.
Figure 2. Strong field ionization yields to the first few cationic doublet states of the molecule starting from different neutral states, S2 (top panel) and S1 (bottom panel), at different geometries (S0 min, S1 min, S2 min, and the S1/S2 CI). 12798
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wavepacket evolution on S1 and S2, which was not possible with earlier experimental measurements. The pump−probe signal for each of the fragments was independently fitted to the convolution of a Gaussian with a sum of two exponentials and a constant:
After considering which ionic states are reached upon ionization, we need to consider fragmentation. While the fragmentation of the molecular cation has been discussed in detail in a previous publication,43 we summarize the main points here. The dissociation energies for the different fragments are given in Table 2. The theoretical values were
f (t ) = e − t
Table 2. Dissociation Energies (DE) in eV for Ionic Fragments of Uracil Relative to the Ground State Minimum of the Cation fragment (amu)
DE exptl44,45
DE calcd43
69 42 41 28
1.3−1.8 4.1 3.8 4.6
1.9 4.0 4.3 4.3
2
/2σ 2
2
⊗ [θ(t − t0) ·(∑ Ai ·e−(t − t0)/ τi + B)] i=1
(1)
where σ parametrizes the impulse response function of our apparatus, limited largely by our pump and probe pulse durations. Here, θ is the Heaviside step function (ensuring that the decay only begins after the excited state has been populated), t0 accounts for different times at which each fragment reaches a maximum, τi are the decay constants and Ai are the amplitudes associated with each of the decays. The constant B can be thought of as an exponential that is too long to be captured in our pump−probe scans. The data was fitted to this function using the Nelder−Mead algorithm46 for multivariate unconstrained nonlinear optimization implemented in MATLAB. The parent ion yield as a function of time as well as the fit are shown in Figure 3. There is excellent agreement between the
obtained at the CCSD(T)/6-311++G(d,p) level by subtracting the energy of the fragments from the parent ion.43 A barrier of about 2 eV to dissociation on the cationic ground state means that ionization to D0 (at least in the FC region) does not lead to dissociation and that the parent ion can be formed. However, ionization to D1, particularly after the molecule has had a chance to acquire kinetic energy on an excited state potential, can lead to dissociation to form the fragment with the lowest dissociation energy: fragment 69 amu. Once an excited molecule relaxes to the ground state, ionization from S0 will lead to the formation of small fragments since the kinetic energy available in S0 after relaxation from S2 is larger than the dissociation energy associated with the production of fragments 28, 41, and 42 amu. Ionization from the S1 minimum means that the wavepacket will have 1 eV of vibrational energy acquired by traveling along the neutral S2 and S1 PESs. Furthermore, ionization from S1 leads to D1, as we see in Figure 2. The energy of D1 at that geometry is 10.48 eV, 1.1 eV above the VIP of 9.4 eV according to our calculations. This energy of 1.1 eV will be available for dissociation. The total energy available for dissociation will be the sum of the vibrational energy acquired in the neutral PES (1 eV) plus the energy acquired during ionization (1.1 eV), which is more than the dissociation energy to form fragment 69, so this fragment can be produced easily. If ionization from the S2 minimum occurs the wavepacket will only have accumulated 0.2 eV of vibrational energy from relaxation along S2. Ionization will lead mostly to D0 with a smaller yield to D1. These states are also 1−1.2 eV higher in energy than their energies in the FC region. Overall, the energy available for dissociation is ca. 1.2−1.4 eV. This energy is likely not enough to lead to dissociation, although some part of the population may dissociate. The considerations above lead us to conclude that ionization from S1 or S0 (following excitation and relaxation from an excited state) leads to dissociation in the cation, and therefore, the parent ion observed has to come from ionization from S2. Thus, the parent ion yields contain information about the evolution of the excited state wavepacket on S2. In contrast, fragment 69 provides information about the wavepacket evolution on both S2 and S1, but not S0, given that ionization from a hot S0 (i.e., excitation to S2 followed by relaxation down to the ground state) would lead to fragmentation into 28, 41, or 42. Thus, by looking at the evolution of the parent ion and fragment 69, we can develop a detailed picture of the
Figure 3. Parent ion and fragment 69 yield as a function of pump− probe delay with fits.
data and the fit. The measurements show a rapid rise and decay (time constant of 70 fs) near the zero time delay, followed by a long decay taking place over several picoseconds (time constant 2.4 ps). The rapid decay in the signal after the zero time delay is consistent with rapid motion away from the Franck−Condon region or a portion of the wavepacket making a rapid nonadiabatic transition to S1 or S0 in less than 100 fs. Any conversion of potential to kinetic energy (either on the same PES or via internal conversion to a lower lying state) can lead to a decay in the ionization yield due to the increase in the ionization potential. Furthermore, the additional kinetic energy acquired by the wavepacket can lead to dissociation in the final ionic state, which can further decrease the parent ion yield at short times. It is the long-lived component of the parent signal that we focus on here since it indicates that a portion of the wavepacket remains trapped near the minimum on S2 for several picoseconds. In the absence of a barrier, the decay from S2 would proceed much more rapidly. The trapping of a portion of the wavepacket on S2 is consistent with some 12799
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previous calculation results, which showed a minimum on S2 near the FC region, as well as previous pump−probe measurements.14,24 The ion yield for fragment 69 amu as a function of time and the associated fit are also shown in Figure 3. As with the parent ion, there is a sharp rise and decay near zero time delay (time constant 90 fs), followed by a long-lived decay (time constant 2.6 ps). In contrast with the parent ion signal, there is also a ledge out toward the longest delay times measured in our experiment, consistent with a decay time much longer than 10 ps. Since the parent ion signal, which comes from ionizing the portion of the wavepacket in S2, decays to zero at long times, the nonzero 69 ion yield at long delays cannot be related to dynamics in S2. However, since any molecules ionized from the ground state after relaxation from S2 would dissociate to form fragments 28, 41, or 42, the 69 signal at long delay times also cannot be due to dynamics on S0. Therefore, the 69 signal at long delay times must be due to dynamics on S1, and we therefore associate this signal at long times with the S1 minimum. Both the S1 and S2 trapping states have been discussed in previous publications. However, although the S1 trapping has been more or less universally accepted, the S2 trapping has been debated more. The reason for this is the great discrepancy between different electronic structure methods to calculate a consistent barrier. This is evident in Table 3 where the value of
spending a few ps trapped near the minimum on S2 and another portion moving fast through the S2/S1 CI. This second portion further branches after passing through the CI and part of it is trapped near the minimum on S1, while another part can reach the S1/S0 CI and decay to the ground state. If the evolution on S2 and S1 proceeded purely sequentially, then we would observe a rise in the 69 signal mirroring the decay of the parent ion. However, both signals rise near time zero with a very small delay between their maxima (less than 50 fs). Our results for the first time give an experimental observable that can be directly related to the S2 population, allowing us to experimentally verify the existence of a barrier on the S2 surface. Population trapped on S1 can spend a long time there, much more than 10 ps, based on our measurements. This is consistent with other measurements of ns lifetimes for the S1 state.6,7 The fact that the parent and 69 signals also have a fast decay component is also consistent with a portion of the wavepacket going rapidly to S0, yielding a picture of multiple bifurcations in the relaxation dynamics.
Table 3. S2 Barriers for Different Levels of Theory
Corresponding Authors
method
barrier (eV)
SA-3-CASSCF(12,9)/6-311+G(d) MRCI(12,9)/6-311+G(d) EOM-CCSD/6-311+G(d) SA-3-CAS(8,6)//6-31G(d)14 SA-5-CAS(8,7)//6-31G(d)14 SA-5-CAS(8,7)-MSPT2//6-31G(d)14 MS-CASPT2(12,9)/DZP25
0.31 −0.09 −0.13 0.26 0.88 0.17
