Ultrafast dynamics of cyclohexene and cyclohexene-d10 excited at 200 nm.

By exciting cyclohexene in the gas phase at 200 nm and probing it by nonresonant multiphoton ionization with mass-selective detection of the ion yields, we found four time constants tau(i) (20, 47, 43, 350 fs). Whereas deuteration lengthens tau2 by a factor of 1.4, the other constants do not change. Tau1-tau3 represent traveling times through observation windows on excited surfaces, whereas tau4 reflects a process in the hot ground state. We assign tau1 (20 fs) to departure from the Franck-Condon regions of the Rydberg and pipi* states, which are both populated at 200 nm, and tau2 (47 fs) to traveling along the pipi* surface and suggest that a [1,3]-sigmatropic H shift begins in this state. This rationalizes the deuterium effect on tau2. To explain why this window is followed by a process not subject to a D effect, we postulate that the pipi surface is crossed late (i.e., at low energy) by the zwitterionic state Z and that formation of a carbene (the known photochemical product, cyclopentylcarbene) begins there. The corresponding 1,2-shift of a CC bond is then (within tau4 = 350 fs) largely reversed on the ground-state surface, while a smaller part of the carbene forms products such as methylenecyclopentane within the same time. Carbene formation is probably accompanied by some cis-trans isomerization. The wavelength dependence of carbene formation is attributed to a memory for the initially excited state, based on momentum conservation. The processes are most likely typical of simple olefins. The fragmentation pattern showed that butadiene is not formed until at least 500 ps. The retro-Diels-Alder reaction, known to take place in the ground state, thus only occurs later.     

2-Naphthyl(carbomethoxy)carbene revisited: combination of ultrafast UV-vis and infrared spectroscopic study

Ultrafast laser flash photolysis (310 nm) of methyl 2-napthyldiazoacetate (2-NpCN2CO2CH3) in acetonitrile or cyclohexane produces a diazo excited state which absorbs broadly in the visible region (tau = 300 fs). The decay of the excited diazo compound is accompanied by growth of the vibrationally excited singlet 2-naphthyl(carbomethoxy)carbene ((1)NpCCO2CH3). The singlet carbene absorbs at 360 and 470 nm. In acetonitrile these bands do not decay over 3 ns, but they do decay by approximately 50% of their original intensity in cyclohexane in 3 ns. It is concluded that (1)NpCCO2CH3 has a singlet ground state in acetonitrile but a triplet ground state in cyclohexane. Related experiments reveal a singlet ground state in Freon-113 and chloroform. This interpretation is supported by ultrafast IR spectroscopy, which confirms that only (1)NpCCO2CH3 is formed within 50 ps of the laser pulse rather than a singlet-triplet equilibrium mixture of carbene. The planar singlet relaxes to the preferred perpendicular singlet over a few tens of picoseconds, as evidenced by a red shift of the carbonyl stretching vibration. Although our data agrees with previous studies, its interpretation is somewhat altered.     

Wave packet simulation of nonadiabatic dynamics in highly excited 1,3-dibromopropane

We have conducted wave packet simulations of excited-state dynamics of 1,3-dibromopropane (DBP) with the aim of reproducing the experimental results of the gas-phase pump-probe experiment by Kotting et al. [ Kotting, C. ; Diau, E. W.-G. ; S?lling, T. I. ; Zewail, A. H. J. Phys. Chem. A 2002, 106, 7530 ]. In the experiment, DBP is excited to a Rydberg state 8 eV above the ground state. The interpretation of the results is that a torsional motion of the bromomethylene groups with a vibrational period of 680 fs is activated upon excitation. The Rydberg state decays to a valence state, causing a dissociation of one of the carbon bromine bonds on a time scale of 2.5 ps. Building the theoretical framework for the wave packet propagation around this model of the reaction dynamics, the simulations reproduce, to a good extent, the time scales observed in the experiment. Furthermore, the simulations provide insight into how the torsion motion influences the bond breakage, and we can conclude that the mechanism that delays the dissociation is solely the electronic transition from the Rydberg state to the valence state and does not involve, for example, intramolecular vibrational energy redistribution (IVR).     

Femtosecond dynamics of isolated phenylcarbenes

Understanding the primary photophysical processes in molecules is essential for interpreting their photochemistry, because molecules rarely react from the initially excited electronic state. In this study the ultrafast excited-state dynamics of chlorophenylcarbene (CPC) and trifluoromethylphenylcarbene (TFPC), two species that are considered as models for carbene dynamics, were investigated by femtosecond time-resolved pump probe spectroscopy in the gas phase. Their dynamics was followed in real time by time-resolved photoionization and photoelectron imaging. CPC was excited at 265 nm into the 3 1A' state, corresponding to excitation from a pi-orbital of the aromatic ring into the LUMO. The LUMO contains a contribution of the p-orbital at the carbene center. Three time constants are apparent in the photoelectron images: A fast decay process with tau1 approximately 40 fs, a second time constant of tau2 approximatley 350 fs, and an additional time constant of tau3 approximately 1 ps. The third time constant is only visible in the time-dependence of low kinetic energy electrons. Due to the dense manifold of excited states between 3.9 and 5 eV, known from ab initio calculations, the recorded time-resolved electron images show broad and unstructured bands. A clear population transfer between the states thus can not directly be observed. The fast deactivation process is linked to either a population transfer between the strongly coupled excited states between 3.9 and 5.0 eV or the movement of the produced wave packet out of the Franck-Condon region. Since the third long time constant is only visible for photoelectrons at low kinetic energy, evidence is given that this time constant corresponds to the lifetime of the lowest excited A 1A' state. The remaining time constant reflects a deactivation of the manifold of states in the range 3.9-5.0 eV down to the A 1A' state.     

Coherent polyatomic dynamics studied by femtosecond time-resolved photoelectron spectroscopy: dissociation of vibrationally excited CS2 in the 6s and 4d Rydberg states

The dissociation dynamics of the 6s and 4d Rydberg states of carbon disulfide (CS(2)*) are studied by time-resolved photoelectron spectroscopy. The CS(2) is excited by two photons of 267 nm (pump) to the 6s and 4d Rydberg states and probed by ionization with either 800 or 400 nm. The experiments can distinguish and successfully track the time dynamics of both spin [1/2] (upper) and [3/2] (lower) cores of the excited Rydberg states, which are split by 60 meV, by measuring the outgoing electron kinetic energies. Multiple mode vibrational wave packets are created within the Rydberg states and observed through recurrence interferences in the final ion state. Fourier transformation of the temporal response directly reveals the coherent population of several electronic states and vibrational modes. The composition of the wave packet is varied experimentally by tuning the excitation frequency to particular resonances between 264 and 270 nm. The work presented here shows that the decay time of the spin components exhibits sensitivity to the electronic and vibrational states accessed in the pump step. Population of the bending mode results in an excited state lifetime of as little as 530 fs, as compared to a several picosecond lifetime observed for the electronic origin bands. Experiments that probe the neutral state dynamics with 400 nm reveal a possible vibrationally mediated evolution of the wave packet to a different Franck-Condon window as a consequence of Renner-Teller splitting. Upon bending, symmetry lowering from D(infinityh) to C(2v) enables ionization to the CS(2) (+) (B (2)Pi(u)) final state. The dissociation dynamics observed are highly mode specific, as revealed by the frequency and temporal domain analysis of the photoelectron spectra.     

Ultrafast dynamics of photochemical radical formation from

The excited-state dynamics and photochemistry of [Re(R)(CO)3(dmb)] (R=Me, Et); dmb=4,4'-dimethyl-2,2'-bipyridine) in CH2Cl2 have been studied by time-resolved visible absorption spectroscopy on a broad time scale ranging from approximately 400 fs to a few microseconds, with emphasis on the femtosecond and picosecond dynamics. It was found that the optically prepared Franck-Condon 1MLCT (singlet metal-to-ligand charge transfer) excited state of [Re(R)(CO)3(dmb)] undergoes femtosecond branching between two pathways (< or =400 fs for R=Me; approximately 800 fs for R=Et). For both methyl and ethyl complexes, evolution along one pathway leads to homolysis of the Re-R bond via a 3SBLCT (triplet sigma-bond-to-ligand charge transfer) excited state, from which [Re(S)(CO)3(dmb)]* and R* radicals are formed. The other pathway leads to an inherently unreactive 3MLCT state. For [Re(Me)(CO)3(dmb)], the 3MLCT state lies lowest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The reactive 3SBLCT state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between the two pathways. [Re(Et)(CO)3(dmb)] behaves differently: branching of the Franck-Condon state between two pathways still occurs, but the 3MLCT excited state lies above the dissociative 3SBLCT state and can decay into it. This shortens the 3MLCT lifetime to 213 ps in CH2Cl2 or 83 ps in CH3CN. Once populated, the 3SBLCT state evolves toward radical photoproducts [Re(S)(CO)3(dmb)]* and Et*. Thus, population of the 3MLCT excited state of [Re(Et)(CO)3(dmb)] provides a second, delayed pathway to homolysis. Hence, the quantum yield is unity. The photochemistry and excited-state dynamics of [Re(R)(CO)3(dmb)] (R=Me, Et) complexes are explained in terms of the relative ordering of the Franck-Condon, 3MLCT, and 3SBLCT states in the region of vertical excitation and along the Re-R reaction coordinate. A qualitative potential energy diagram is proposed.     

Ultrafast UV-vis and IR studies of p-biphenylyl acetyl and carbomethoxy carbenes

The photochemistry of a p-biphenylyl diazo ester (BpCN2CO2CH3) and diazo ketone (BpCN2COCH3) were studied by ultrafast time-resolved UV-vis and IR spectroscopies. The excited states of these diazo compounds were detected and found to decay with lifetimes of less than 300 fs. The diazo ester produces singlet carbene with greater quantum efficiency than the ketone analogue due to competing Wolff rearrangement (WR) in the excited state of the diazo ketone. Carbene BpCCO2CH3 has a singlet-triplet gap that is close to zero in cyclohexane, but the triplet is the ground state. The two spin states are in rapid equilibrium in this solvent relative to reaction with cyclohexane. There is (for a carbene) a slow rate of singlet to triplet intersystem crossing (isc) in this solvent because the orthogonal singlet must rotate to a higher energy orientation prior to isc. In acetonitrile and in dichloromethane BpCCO2CH3 has a singlet ground state. Ketocarbene BpCCOCH3 has a singlet ground state in cyclohexane, in dichloromethane, and in acetonitrile and decays by WR to form a ketene detected by ultrafast IR spectroscopy in these solvents. Ketocarbenes have more stable singlet states, relative to carbene esters, because of the superior conjugation of the filled hybrid orbital of the carbene with the pi system of the carbonyl group, the same factor that makes methyl ketones more acidic than the analogous esters. The rate of WR of BpCCOCH3 is faster in cyclohexane than in dichloromethane and acetonitrile because of intimate solute-solvent interactions between the empty p orbital of the carbene and nonbonding electron pairs of heteroatoms of the solvent. These interactions stabilize the carbene and retard the rate of WR.     

A study of the photochemistry of Diazo Meldrum's acid by ultrafast time-resolved spectroscopies

The photochemistry of Diazo Meldrum's acid (DM) was investigated by fs time-resolved UV-vis and IR spectroscopic methods. UV (266 nm) excitation of DM pumps the molecule to the S 5 and S 7 excited states. After fast internal conversion (IC), the S 2 state is formed, which will undergo Wolff rearrangement to form vibrationally excited ketene, which relaxes in 9 ps. The S 2 state will also relax to the S 1 state, which isomerizes to diazirine, fragments to form carbene, and relaxes further to the ground state of DM. The singlet carbene absorbs at 305 nm, is formed within 300 fs of the laser pulse, and has a lifetime of 2.3 ps in acetonitrile. The lifetime of DM in the S 2 and S 1 states is less than 300 fs. The quantum efficiency of DM decomposition is approximately 50% in chloroform with 266 nm excitation.     

飞秒时间分辨的光电子影像对3-甲基吡啶分子超快动力学研究

利用飞秒时间分辨的光电子影像技术结合时间分辨的质谱技术,研究了3-甲基吡啶分子激发态的超快过程. 实时观察到了3-甲基吡啶分子S₂态向S₁态高振动能级的超快内转换过程,该内转换的时间大约为910fs. 二次布居的S₁态主要通过内转换衰减到基态S₀,该内转换的时间尺度为2.77 ps. 光电子能谱分布和光电子角分布显示,S₂态和S₁态在电离的过程中跟3p里德堡态发生偶然共振. 本次实验中还用400 nm两个光子吸收的方法布居了3-甲基吡啶的3s 里德堡态. 研究表明,3s 里德堡态的寿命为62 fs,并主要通过内转换快速衰减到基态.     

First principles simulation of the UV absorption spectrum of ethylene using the vertical Franck-Condon approach

A new method which we refer to as vertical Franck-Condon is proposed to calculate electronic absorption spectra of polyatomic molecules. In accord with the short-time picture of spectroscopy, the excited-state potential energy surface is expanded at the ground-state equilibrium geometry and the focus of the approach is more on the overall shape of the spectrum and the positions of the band maxima, rather than the precise position of the 0-0 lines. The Born-Oppenheimer approximation and the separability of the excited-state potential energy surface along the excited-state normal mode coordinates are assumed. However, the potential surface is not necessarily approximated as harmonic oscillator potentials along the individual normal modes. Instead, depending upon the nature of the potential surface along a particular normal mode, it is treated either in the harmonic approximation or the full one-dimensional potential is considered along this mode. The vertical Franck-Condon approach is applicable therefore even in cases where the excited state potential energy surface is highly anharmonic and the conventional harmonic Franck-Condon approach is inadequate. As an application of the method, the ultraviolet spectrum of ethylene between 6.2 eV (50,000 cm(-1)) and 8.7 eV (70,000 cm(-1)) is simulated, using the Similarity Transformed Equation of Motion Coupled-Cluster method to describe the required features of the potential energy surfaces. The spectrum is shown to be a result of sharp doublet structures stemming from the pi --> 3s (Rydberg) state superimposed on top of a broad band resulting from the pi --> pi* (valence) state. For the Rydberg state, the symmetric C=C stretch and the torsion mode contribute to the spectrum, while the broad valence band results from excitation into the C=C stretch, CH2 scissors, and the torsion mode. For both states, the potential along the torsion mode is highly anharmonic and the full treatment of the potential along this mode in the vertical Franck-Condon method is required.     

A 4D wave packet study of the CH3I photodissociation in the A-band. Comparison with femtosecond velocity map imaging experiments

The time-resolved photodissociation dynamics of CH(3)I in the A-band has been studied theoretically using a wave packet model including four degrees of freedom, namely the C-I dissociation coordinate, the I-CH(3) bending mode, the CH(3) umbrella mode, and the C-H symmetric stretch mode. Clocking times and final product state distributions of the different dissociation (nonadiabatic) channels yielding spin-orbit ground and excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch ν(1) and umbrella ν(2) modes) CH(3) fragments have been obtained and compared with the results of femtosecond velocity map imaging experiments. The wave packet calculations are able to reproduce with very good agreement the experimental reaction times for the CH(3)(ν(1), ν(2))+I*((2)P(1/2)) dissociation channels with ν(1) = 0 and ν(2) = 0,1,2, and also for the channel CH(3)(ν(1) = 0, ν(2) = 0)+I((2)P(3/2)). However, the model fails to predict the experimental clocking times for the CH(3)(ν(1), ν(2))+I((2)P(3/2)) channels with (ν(1), ν(2)) = (0, 1), (0, 2), and (1, 0), that is, when the CH(3) fragment produced along with spin-orbit ground state I atoms is vibrationally excited. These results are similar to those previously obtained with a three-dimensional wave packet model, whose validity is discussed in the light of the results of the four-dimensional treatment. Possible explanations for the disagreements found between theory and experiment are also discussed.     

Femtosecond dynamics of cyclopropenylidene, c-C3H2

The photophysics of the B (1)B(1) state of isolated cyclopropenylidene, c-C(3)H(2), has been studied by femtosecond time-resolved photoionisation and photoelectron spectroscopy. The carbene was produced by flash pyrolysis of 3-chlorocycloprop-1-ene. The bands at 266.9 nm and 264.6 nm have been investigated. The excited state deactivates in a two step process. The first time constant of less than 50 fs corresponds most likely to a nonradiative transition to the A-state, the second one on the order of 200 fs describes the internal conversion to the electronic ground state. The data are compared to those measured for the chlorinated carbene c-C(3)HCl. In the photoelectron spectrum of c-C(3)H(2) resonances were observed which can be assigned to members of a Rydberg d-series.     

Large dynamic Stokes shift of DNA intercalation dye Thiazole Orange has contribution from a high-frequency mode

Fluorescence of the cyanine dye Thiazole Orange (TO) is quenched by intramolecular twisting in the excited state. In polypeptide nucleic acids, a vibrational progression in a 1400 cm(-1) mode depends on base pairing, from which follows that the high-frequency displacement is coupled to the twist coordinate. The coupling is intrinsic to TO. This is shown by femtosecond fluorescence upconversion and transient absorption spectroscopy with the dye in methanol solution. Narrow emission from the Franck-Condon state shifts to the red and broadens within 100 fs. The radiative rate does not decrease during this process. Vibrational structure builds up on a 200 fs time scale; it is assigned to asymmetric stretching activity in the methine bridge. Further Stokes shift and decay are observed over 2 ps. Emission from the global S(1) minimum is discovered in an extremely wide band around 12 000 cm(-1). As the structure twists away from the Franck-Condon region, the mode becomes more displaced and overlap with increasingly higher vibrational wave functions of the electronic ground state is achieved. Twisting motion is thus leveraged into a fast-shrinking effective energy gap between the two electronic states, and internal conversion ensues.     

Effects of benzoannulation and alpha-octabutoxy substitution on the photophysical behavior of nickel phthalocyanines: a combined experimental and DFT/TDDFT study

The photophysical properties of a group of Ni(II)-centered tetrapyrroles have been investigated by ultrafast transient absorption spectrometry and DFT/TDDFT methods in order to characterize the impacts of alpha-octabutoxy substitution and benzoannulation on the deactivation pathways of the S1(pi,pi*) state. The compounds examined were NiPc, NiNc, NiPc(OBu)8, and NiNc(OBu)8, where Pc = phthalocyanine and Nc = naphthalocyanine. It was found that the S1(pi,pi*) state of NiNc(OBu)8 deactivated within the time resolution of the instrument (200 fs) to a vibrationally hot T1(pi,pi*) state. The quasidegeneracy of the S1(pi,pi*) and 3(dz2,dx2-y2) states allowed for fast intersystem crossing (ISC) to occur. After vibrational relaxation (ca. 2.5 ps), the T1(pi,pi*) converted rapidly (ca. 19 ps lifetime) and reversibly into the 3LMCT(pi,dx2-y2) state. The equilibrium state, so generated, decayed to the ground state with a lifetime of ca. 500 ps. Peripheral substitution of the Pc ring significantly modified the photodeactivation mechanism of the S1(pi,pi*) by inducing substantial changes in the relative energies of the S1(pi,pi*), 3(dpi,dx2-y2), 3(dz2,dx2-y2), T1(pi,pi*), and 1,3LMCT(pi,dx2-y2) excited states. The location of the Gouterman LUMOs and the unoccupied metal level (dx2-y2) with respect to the HOMO is crucial for the actual position of these states. In NiPc, the S1(pi,pi*) state underwent ultrafast (200 fs) ISC into a hot (d,d) state. Vibrational cooling (ca. 20 ps lifetime) resulted in a cold (dz2,dx2-y2) state, which repopulated the ground state with a 300 ps lifetime. In NiPc(OBu)8, the S1(pi,pi*) state deactivated through the 3(dz2,dx2-y2), which in turn converted to the 3LMCT(pi,dx2-y2) state, which finally repopulated the ground state with a lifetime of 640 ps. Insufficient solubility of NiNc in noncoordinating solvents prevented transient absorption data from being obtained for this compound. However, the TDDFT calculations were used to make speculations about the photoproperties.     

Role of the intermolecular vibrations in the hydrogen transfer rate: the 3-methylindole-NH3 complex

The lifetimes of the 3-methylindole-NH3 complex have been measured on different vibronic levels involving intermolecular modes and decrease from 530 to 65 ps, in a mode specific manner. Geometry optimizations of the ground and excited states have been performed with ab initio methods, and as in the case of phenol and indole, a repulsive pi sigma* state lies close to the initially excited pi pi* state. From these calculations, it seems that both in-plane and out-of-plane vibrations induce a faster nonradiative decay.     

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH(3)I

The real time photodissociation dynamics of CH(3)I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH(3) fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH(3)(nu=0) and I((2)P(32)) and I(*)((2)P(12)) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I(*) channels when the CH(3) fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model.     

Spectroscopy and femtosecond dynamics of the ring opening reaction of 1,3-cyclohexadiene

The early stages of the ring opening reaction of 1,3-cyclohexadiene to form its isomer 1,3,5-hexatriene, upon excitation to the ultrashort-lived 1 1B2 state, were explored. A series of one-color two-photon ionization/photoelectron spectra reveal a prominent vibrational progression with a frequency of 1350 cm(-1), which is interpreted in a dynamical picture as resulting from the ultrafast wave packet dynamics associated with the ring opening reaction. Photoionization in two-color three-photon and one-color four-photon ionization schemes show an ionization pathway via the same ultrashort-lived 1 1B2 state, and in addition, a series of Rydberg states with quantum defects of 0.93, 0.76, and 0.15, respectively. Using those Rydberg states as probes for the reaction dynamics in a time-resolved pump-probe experiment provides a direct observation of the elusive 2 1A1 state that has been implicated as an intermediate step between the initially excited 1 1B2 state and the ground electronic state. The rise and decay times for the 2 1A1 state were found to be 55 and 84 fs, respectively.     

Quantum dynamics study of fulvene double bond photoisomerization: the role of intramolecular vibrational energy redistribution and excitation energy

The double bond photoisomerization of fulvene has been studied with quantum dynamics calculations using the multi-configuration time-dependent Hartree method. Fulvene is a test case to develop optical control strategies based on the knowledge of the excited state decay mechanism. The decay takes place on a time scale of several hundred femtoseconds, and the potential energy surface is centered around a conical intersection seam between the ground and excited state. The competition between unreactive decay and photoisomerization depends on the region of the seam accessed during the decay. The dynamics are carried out on a four-dimensional model surface, parametrized from complete active space self-consistent field calculations, that captures the main features of the seam (energy and locus of the seam and associated branching space vectors). Wave packet propagations initiated by single laser pulses of 5-25 fs duration and 1.85-4 eV excitation energy show the principal characteristics of the first 150 fs of the photodynamics. Initially, the excitation energy is transferred to a bond stretching mode that leads the wave packet to the seam, inducing the regeneration of the reactant. The photoisomerization starts after the vibrational energy has flowed from the bond stretching to the torsional mode. In our propagations, intramolecular energy redistribution (IVR) is accelerated for higher excess energies along the bond stretch mode. Thus, the competition between unreactive decay and isomerization depends on the rate of IVR between the bond stretch and torsion coordinates, which in turn depends on the excitation energy. These results set the ground for the development of future optical control strategies.     

Energy flow and fragmentation dynamics of n,n-dimethylisopropylamine

The energy flow and fragmentation dynamics of N,N-dimethylisopropylamine (DMIPA) upon excitation to the 3p Rydberg states has been investigated with use of time-resolved photoelectron and mass spectrometry. The 3p states are short-lived, with a lifetime of 701 +/- 45 fs. From the time dependence of the photoelectron spectra, we infer that the primary reaction channel leads to the 3s level, which itself decays to the ground state with a decay time of 87.9 +/- 10.2 ps. The mass spectrum reveals fragmentation with cleavage at the alpha C-C bond, indicating that the energy deposited in vibrations during the internal conversion from 3p to 3s exceeds the bond energy. A thorough examination of the binding energies and temporal dynamics of the Rydberg states, as well as a comparison to the related fragmentation of N,N-dimethyl-2-butanamine (DM2BA), suggests that the fragments are formed on the ion surfaces, i.e., after ionization and on a time scale much slower than the fluorescence decay from 3s to the ground state.