Ultrafast Photodissociation

In Chap. 3, wave packet propagation could be observed for nearly all of the alkali dimer and trimer systems considered, over a rather long time compared to the wave packet oscillation period. The wave packet dynamics - a fingerprint of the excited molecule - definitely characterize the excited bound electronic state of these molecules. However, with the results on K3 (excited with A 800 nm), another phenomenon, which often governs ultrafast molecular and cluster dynamics, comes into the discussion photodissociation induced by the absorption of single photons. This photoinduced dissociation permits detailed study of molecular dynamics such as breaking of bonds, internal energy transfer, and radiationless transitions. The availability of laser sources with pulses of a few tens of femtoseconds today opens a direct, i.e. real-time, view on this phenomenon. 

In Fig. 4.1 mass spectra of Na =3.. 9 excited with laser pulses of sub-50 fs length, measured for three different delay times At, are shown. It is clearly visible that for each cluster size the ion intensity is dramatically reduced 

This type of ultrafast dynamics will now be investigated in detail for several alkali aggregates, beginning with the model molecule Nas excited to its C state (Sect. 4.1) and D state (Sect. 4.2). The ultrafast fragmentation of sodium clusters (Sect. 4.3) and potassium clusters (Sect. 4.4) rounds off these studies. 

The sodium trimer excited to the electronic C state can be regarded as a fascinating model system, which manifests ultrafast predissociation dynamics. While stationary and nanosecond-pump probe spectroscopy gave the first hints that this excited state photodissociates rather fast, real-time TPI spectroscopy opens a window to directly observe these ultrafast processes. But let us first start with a short review of the spectroscopy of this excited electronic state. 

Spectroscopic Basics of Nas C. In the late 1980s special interest was focused on the C(2) E state (in Dsh symmetry) of Nas. Energy-resolved spectroscopy allowed the observation of lower vibrational levels of this electronic state by means of TPI, whereas the upper levels require the use of DS to probe dissociative states [369, 374, 393]. The spectrum of the C state is characterized by a vibrational band structure with pseudorotational features, as shown in Fig. 4.3. These investigations confirmed the C state to be partially predissociated. Therefore, the dissociation channel was proposed to be the main relaxation process for states higher in energy than the C state. This could also be demonstrated for the D state by the depletion technique with a few nanoseconds time resolution [375], as well as for Rydberg states close to the ionization limit [124]. 

---

Fig. 4 Coherent oscillations observed by Fup and co-workers after ultrafast photodissociation of group 6 metal carbonyls. The Fourier transform of the oscillatory part shows a peak at 96cm which compares to 98cm found for equatorial L-M-L bending in Jahn-Teller moat using semi-classical direct dynamics [43] (reused from [70] with permission)...

Photochemical studies of simple mononuclear metal carbonyls have been relatively few in the period under review. The ultrafast photodissociation dynamics of Cr(CO)6 following excitation at 200 and 267 nm have been... 

Banin U, Waldman A and Ruhman S J 1992 Ultrafast photodissociation of l in solution direct observation of coherent product vibrations J. Chem. Phys. 96 2416-19... 

Banin U and Ruhman S 1993 Ultrafast photodissociation of 1.. Coherent photochemistry in solution J. Chem. Phys. 98 4391-403... 

To observe ultrafast fragmentation of excited alkali clusters the appropriate tool is real-time MPI spectroscopy. This technique allows the mass-selected detection of the ultrafast photodissociation with high sensitivity. In 1992 Gerber and coworkers presented the first femtosecond time-resolved experiments in cluster physics [32, 131, 132], showing differences in the fragmentation behavior of Nan<2i clusters dependent on the excitation at different wavelengths. 

In the second part of this chapter (Sect. 3.2), different wave packet propagation phenomena in excited alkali trimers are discussed. The time-resolved pseudorotation of the sodium trimer is presented in Sect. 3.2.2. Last but not least, applying laser pulses of the same wavelength but of different pulse width enables a mode-selective preparation of the trimer, hence controlling its dynamics (Sect. 3.2.4). Wave packet propagation on a repulsive PES (Sect. 3.2.5), studied on the potassium trimer, leads to the phenomena of ultrafast photodissociation, which then is the topic of the subsequent chapter. 

Ultrafast Photodissociation. Since radiative decay of the trimer is in the time domain of nanoseconds, this population can be changed either by photodissociation or by intersystem crossing to an electronic state, from which, under these experimental conditions, no ionization can take place. However, in the latter case this behavior should be directly visible as a drastic modulation of the observed wave packet propagation [43, 329, 315]. But this is not found. Apart from this, the first theoretical calculations of the PES of Ka gave no evidence of the existence of any dark state in this energy regime. Therefore, it seems to be reasonable that ultrafast photodissociation causes the fast decay of the ion signal. The K2 can also be ionized by the probe pulse. 

The ultrafast photodissociation dynamics of the Na3 C state was analyzed with time-resolved two-color TPI spectroscopy in the picosecond regime. The two excitation wavelengths required, independently tunable for the pump and the probe pulse, were generated by a home-built synchronization of two mode-locked titanium sapphire lasers. The deconvoluted real-time spectra can be well described by a single exponential decay with a time constant strongly... 

For additional studies of the ultrafast photodissociation of small alkali clusters, the support of detailed qualitative and quantitative predictions of the stability and photoinduced fragmentation of these many-particle systems is essential. Knowing that even for the three-body system Nas, state-of-the-art time-dependent quantum dynamical simulations are extremely costly, requiring considerable computer time, the application of other concepts is necessary. In the near future, the density matrix formalism might be an appropriate approach here. 

Ultrafast photodissociation dynamics, investigated for larger, very cold alkali aggregates Mnz=3...io (M = Na, K) as a function of excitation energy and cluster size. This allows detailed information on the stability of small alkali aggregates. 

Trushin SA, Fuss W, Schmid WE (2000) Conical intersections, pseudorotation and coherent oscillations in ultrafast photodissociation of group-6 metal hexacarbonyls. Chem Phys 259 313-330... 

Figure 8.3 A schematic two-dimensional view of the potential energy surface and wave-packet dynamics in the ultrafast photodissociation of Hgl2 [adapted from Voth and Hochstrasser (1996), Zewail (1996)]. The transition state for the I + Hgl reaction is along the bisector, dashed line, with the lowest barrier at the bottom of the potential along that line. The UV excitation creates a localized wave-packet along the bisector. The center of the packet is displaced from the saddle point to a compressed configuration along the symmetric stretch. During the dissociation the wave-packet bifurcates, as shown, and each component is followed in the figure. It shows the coherent vibrational motion in the Hg—I well. ...

Coherence in the nascent products is one manifestation of the limitation of a one-dimensional point of view because it refers to motion not along the reaction coordinate. Will the coherence survive for more complex systems or for situations such as reactions in solution where the coupling to the environment is a key The answer to both questions is yes. In solution, the coherence of Hg—I motion following ultrafast photodissociation of Hgl2 is observable. Figure 8.4. 

Farmanara, P., V. Stert, and W. Radloff (1999), Ultrafast photodissociation of methyl nitrite excited to the S2 state, Chem. Phys. Lett., 303, 521-528. 

---