We report the results of theoretical studies of the time-resolved femtosecond photoelectron spectroscopy of quantum wavepackets through the conical intersection between the first two 2 AЈ states of NO 2 . The Hamiltonian explicitly includes the pump-pulse interaction, the nonadiabatic coupling due to the conical intersection between the neutral states, and the probe interaction between the neutral states and discretized photoelectron continua. Geometry-and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Photoelectron angular distributions are seen to provide a clearer picture of the ionization channels and underlying wavepacket dynamics around the conical intersection than energy-resolved spectra. Time-resolved photoelectron velocity map images are also presented.

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Articles you may be interested inComment on "Photoelectron angular distributions as a probe of alignment in a polyatomic molecule: Picosecond time-and angle-resolved photoelectron spectroscopy of S1 p-difluorobenzene" [J. Chem. Phys.111, 1438 (1999 We present a formulation of energy-and angle-resolved photoelectron spectra for femtosecond pump-probe ionization of wave packets and results of its application to the 1 ⌺ u ϩ double-minimum state of aligned Na 2 . The formulation is well-suited for inclusion of the underlying dynamics of molecular photoionization and its dependence on molecular geometry. Results are presented for three typical pump laser energies selected so as to investigate qualitatively different patterns of the spatio-temporal propagation of wave packets on the double-minimum potential curve and of their associated photoelectron spectra. Photoelectron angular distributions are also reported for different orientations of linearly polarized pump and probe pulses. The resulting photoelectron spectra illustrate the importance of a proper description of the underlying photoionization amplitudes and their dependence on geometry for unraveling wave packet dynamics from pump-probe photoelectron signals in nonadiabatic regions where the electronic structure evolves rapidly with geometry. The dependence of these photoelectron angular distributions on relative orientation of the molecule and polarization of the probe pulse are also seen to be potentially useful for real-time monitoring of molecular rotation.

Time-resolved photoelectron spectroscopy (TRPES) is a useful approach to elucidate the coupled electronic-nuclear quantum dynamics underlying chemical processes, but has remained limited by the use of low photon energies. Here, we demonstrate the general advantages of XUV-TRPES through an application to NO2, one of the simplest species displaying the complexity of a non-adiabatic photochemical process. The high photon energy enables ionization from the entire geometrical configuration space, giving access to the true dynamics of the system. Specifically, the technique reveals dynamics through a conical intersection, large-amplitude motion and photodissociation in the electronic ground state. XUV-TRPES simultaneously projects the excited-state wave packet onto many final states, offering a multi-dimensional view of the coupled electronic and nuclear dynamics. Our interpretations are supported by ab initio wavepacket calculations on new global potential-energy surfaces. The presented results contribute to establish XUV-TRPES as a powerful technique providing a complete picture of ultrafast chemical dynamics from photoexcitation to the final products.

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The application of femtosecond pump-probe photoelectron spectroscopy to directly observe vibrational wave packets passing through an avoided crossing is investigated using quantum wave packet dynamics calculations. Transfer of the vibrational wave packet between diabatic electronic surfaces, bifurcation of the wave packet, and wave packet construction via nonadiabatic mixing are shown to be observable as time-dependent splittings of peaks in the photoelectron spectra. DOI: 10.1103/PhysRevLett.90.248303 PACS numbers: 82.20.Gk, 33.60.-q, 82.53.Eb, 82.53.Kp The concept of nonadiabatic transitions is fundamental to an understanding of chemical phenomena [1,2]. A typical example, and the one of interest here, is intramolecular electron transfer induced by vibrational motion in the excited state of alkali halides such as the NaI molecule. Pump-probe studies of this system have demonstrated the decrease of wave packet population on the excited adiabatic surface due to dissociation at the avoided crossing [3,4]. Oscillations in the population of the dissociative products (Na and I) due to the interference of wave packets on the covalent and ionic potentials merging at the avoided crossing have also been observed [3]. Although bifurcation of wave packets must be invoked to explain these observations, no real-time evidence of the instance of wave packet bifurcation has yet been experimentally observed. Such direct observation, if possible, would be interesting in itself, but also quite significant to studies of electron transfer, wave packet engineering, and reaction control through wave packet splitting and mixing. Also, from the perspective of quantum measurement, wave packet bifurcation corresponding to an intramolecular double-slit experiment will shed light on the evolution of quantum entanglement between electronic and nuclear motion [5]. Bifurcation and merging of wave packets are important as an intrinsic mechanism of ''quantum chaos,'' which has no simple classical counterpart [6 -8].Pump-probe photoelectron spectroscopy has been demonstrated to be a powerful means to monitor real-time reaction dynamics [9][10][11][12][13][14]. We here show theoretically that the method permits real-time observations of a wave packet passing through an avoided crossing, using excited-state dynamics of NaI as an example.The pump-probe spectroscopy of NaI has been studied extensively [3,4,[15][16][17][18][19][20][21]. Figure 1 depicts the pump-probe scheme and the relevant potential curves for femtosecond photoelectron spectroscopy of the excited-state wave packet dynamics. The adiabatic excited state features an extended well whose character changes from covalent at shorter distances to ionic at larger distances due to an avoided crossing with the ground state at 7 A [22,23]. In the diabatic representation, the same system is viewed as an ionic curve (here called V 1 ) intersecting a covalent curve (V 2 ), with an associated nonadiabatic interaction (V 12 ). The diabatic curves are shown in Fig. 1.The excited-state wave packet...

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We have previously shown how femtosecond angle-and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle-and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two 2 A 0 states of NO 2 that is transformed to an avoided crossing. Geometry-and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.