Femtosecond Time-resolved Mass Spectrometry Research Articles

Photodissociation Dynamics of the Highly Stable ortho-Nitroaniline Cation.

0rtho-Nitroaniline (ONA) is a model for the insensitive high explosive 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) that shares strong hydrogen bonding character between adjacent nitro and amino groups. This work reports femtosecond time-resolved mass spectrometry (FTRMS) measurements and theoretical calculations that explain the high stability of the ONA cation compared with related nitroaromatic molecules. Ab initio calculations found that the lowest-lying electronic excited state of the ONA cation, D1, lies more than 2 eV above the ground state, and the energetic barriers to rearrangement and dissociation reactions exceed this D1 energy. These theoretical results were confirmed by FTRMS pump-probe measurements showing that (1) fragment ions represented less than 30% of the total ion yield when a 1014 W cm-2, 1300 nm, 20 fs pump pulse was used to ionize ONA; and (2) 3.1 eV (400 nm) photons were required to induce dissociation of the ONA cation. Stronger coupling between the ground D0 and excited D4 states of the ONA cation at the geometry of neutral ONA resulted in a transient enhancement of fragment ion yields at <300 fs pump-probe delay times, prior to relaxation of the ONA cation to its optimal geometry.

Size onset of metallic behavior in neutral aluminum clusters.

The excited state lifetimes of neutral (Al)n clusters up to ∼1 nm in diameter in size, where n ≤ 43, are systematically measured with femtosecond time-resolved mass spectrometry. The onset of metallic behavior is identified as a distinct change in the relaxation behavior initiated with single ultraviolet (400 nm) photon excitation. The experimentally measured excited state lifetimes gradually decrease with size for small molecular scale clusters (n < 10) before becoming indistinguishable for larger clusters (n > 9), where the measurements are comparable to electron-lattice relaxation time of bulk Al (∼300 fs). Particularly intense, or magic, Aln clusters do not exhibit any significant excited state lifetime behavior. Time-dependent density functional theory quantify the excited state properties and are presented to show that dynamics are strongly tied to the excited state charge carrier distributions and overlap, rather than detailed changes related to changes in the cluster's electronic and geometric structure. The consistency in excited state lifetimes for clusters larger than n = 9 is attributed to the hybridization of the s- and p-orbitals as well as increasing delocalization. Al3 exhibits unique temporal delay in its transient behavior that is attributed to a transition from triangular ground state to linear structure upon excitation.

Ultrafast Proton Transfer and Contact Ion-Pair Formation in Formic Acid Clusters.

The ultrafast proton transfer dynamics of homogeneous formic acid clusters (FA)n, n < 10, are investigated with femtosecond time-resolved mass spectrometry. We monitor the proton transfer pathway following Rydberg state electronic relaxation and find that successful ion pair formation increases logarithmically with cluster size. Ab initio calculations demonstrate similar excitation/relaxation behavior for each cluster, revealing a contact ion pair forms between two molecules composing the cluster before finally a formate anion (HCOO-) is dissociated by the probe pulse. The sub-ps time scale for rearrangement and proton transfer increases almost linearly with cluster size, requiring ∼67 fs per additional formic acid molecule and ranging from 213 ± 51 fs for the trimer to 667 ± 116 fs for FA9. The near-linear trends measured for both rearrangement lifetime and ion pair formation suggest that proton transfer is unlikely in the formic acid dimer but becomes prominent in small clusters.

Nonadiabatic Photodissociation and Dehydrogenation Dynamics of n-Butyl Bromide Following p-Rydberg Excitation.

Femtosecond time-resolved mass spectrometry, correlation mapping, and density functional theory calculations are employed to reveal the mechanism of C═C and C≡C formation (and related H2 production) following excitation to the p-Rydberg states of n-butyl bromide. Ultrafast pump-probe mass spectrometry shows that nonadiabatic relaxation operates as a multistep process reaching an intermediate state within ∼500 fs followed by relaxation to a final state within 10 ps of photoexcitation. Absorption of three ultraviolet photons accesses the dense p-Rydberg state manifold, which is further excited by the probe beam for C─C bond dissociation and dehydrogenation reactions. Rapid internal conversion deactivates the dehydrogenation pathways, while activating carbon backbone dissociation pathways. Thus, unsaturated carbon fragments decay with the lifetime of p-Rydberg (∼500 fs), matching the growth recorded in saturated hydrocarbon fragments. The saturated hydrocarbon signals subsequently decay on the picosecond time scale as the molecule relaxes below the Rydberg states and into halogen release channels.

Femtosecond Time-Resolved Observation of Relaxation and Wave Packet Dynamics of the S1 State in Electronically Excited o-Fluoroaniline

Quantum beat frequency is the basis for understanding interference effects and vibrational wave packet dynamics and has important applications. Using femtosecond time-resolved mass spectrometry and femtosecond time-resolved photoelectron image combined with theoretical calculations, we study the electronic excited-state relaxation of o-fluoraniline molecule and the time-dependent evolution of vibrational wave packets between different eigenstates. After the molecule absorbs a photon of 288.3 nm and is excited to the S1 state, intramolecular vibrational redistribution first occurs on the time scale τ1 = 349 fs, and then the transition to the triplet state occurs through the intersystem crossing on the time scale τ2 = 583 ps, and finally, the triplet state occurs decays slowly through the time scale τ3 = 2074 ps. We find the intramolecular vibrational redistribution is caused by the 00, 10b1 and 16a1 vibrational modes of the Sl state origin. That is, the 288.3 nm femtosecond laser excites the molecule to the S1 state, and the continuous flow of the vibrational wave packet prepares a coherent superposition state of three vibrational modes. Through extracting the oscillation of different peak intensities in the photoelectron spectrum, we observe reversible changes caused by mutual interference of the S1 00, S1 10b1 and S1 16a1 states when the wave packets flow. When the pump pulse is 280 nm, the beat frequency disappears completely. This is explained in terms of increases in the vibrational field density and characteristic period of oscillation, and statistical averaging makes the quantum effect smooth and indistinguishable. In addition, the Rydberg component of the S1 state is more clearly resolved by combining experiment and theory.

Ion-Pair Formation in n-Butyl Bromide through 5p Ryberg State Predissociation.

The ultrafast photodynamics of n-butyl bromide are explored with femtosecond time-resolved mass spectrometry. Absorption of two UV (400 nm) pump photons induces the direct dissociation of the C-Br bond from the A state within 160 fs. Absorption of three UV pump photons excites the molecule into the 5p Rydberg state which undergoes several relaxation pathways including to the ion-pair state. Relaxation to the ion-pair state is tracked through the transient of the C4H9+ fragment and suggests an E state lifetime of 10.8 ± 0.5 ps, in close agreement with the tunneling time of smaller molecules. Predissociation from the 5p Rydberg states leads to the β-elimination of H-Br and formation of C4H8+ within 3.0 ± 0.25 ps. A portion of the excited parent molecule avoids the ion-pair formation and instead relaxes through the Rydberg excited state manifold into the D state within 30.2 ± 0.21 ps.

Quantitative Analysis of Nitrotoluene Isomer Mixtures Using Femtosecond Time-Resolved Mass Spectrometry.

Discrimination of isomers in a mixture is a subject of ongoing interest in biology, pharmacology, and forensics. We demonstrate that femtosecond time-resolved mass spectrometry (FTRMS) effectively quantifies mixtures of ortho-, para-, and meta-nitrotoluenes, the first two of which are common explosive degradation products. The key advantage of the FTRMS approach to mixture quantification lies in the ability of the pump-probe laser control scheme to capture distinct fragmentation dynamics of each nitrotoluene cation isomer on femtosecond timescales, thereby allowing for discrimination of the isomers using only the signal of the parent molecular ion at m/z 137. Upon measurement of reference dynamics of each individual isomer, the molar fractions of binary and ternary mixtures can be predicted to within ∼5 and ∼7% accuracy, respectively.

Ultrafast dynamics of electron excited states of phenylacetylene

Interaction of light with matter has always been important in the field of natural science. Particularly, the ultrafast radiationless relaxation induced by UV light of molecular electronic excited states accompanied by ultrafast energy transfer plays an important role in the natural photophysical, photochemical and biological reactions. Generally, the molecular electronic excited state can be deactivated through a variety of decay channels, including dissociation, isomerization, internal conversion, intersysterm crossing, vibrational energy redistribution, and autoionization. This complexity of relaxation channels brings about a wide variety of deactivation mechanisms. The ultrafast nonadibatic relaxation dynamics of the excited state of phenylacetylene is studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S&lt;sub&gt;2&lt;/sub&gt; of phenylacetylene is excited by 235 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The time-dependent curves of parent ions include two exponential curves. One is the fast component with a time constant of 116 fs, and the other is the slow component with a time constant of 106 ps. The time-resolved photoelectron kinetic energy distribution is obtained from the time-resolved photoelectron images. Combined with the time-resolved photoelectron spectroscopy data, the fast component with a time constant of 116 fs is found to reflect the internal conversion process from S&lt;sub&gt;2&lt;/sub&gt; state to S&lt;sub&gt;1&lt;/sub&gt; state. The experimental results also show that S&lt;sub&gt;1&lt;/sub&gt; state is arranged by internal conversion, and the inter system jump process to T&lt;sub&gt;1&lt;/sub&gt; state is an important attenuation channel. This work provides a clearer physical picture for S&lt;sub&gt;1&lt;/sub&gt; state nonadibatic relaxation dynamics of phenylacetylene.

Using computational chemistry to design pump-probe schemes for measuring nitrobenzene radical cation dynamics.

The electronic potential energy surfaces of the nitrobenzene cation obtained from time-dependent density functional theory and coupled cluster calculations are used to predict the most efficient excitation wavelength for femtosecond time-resolved mass spectrometry measurements. Both levels of theory identify a strongly-coupled transition from the ground state of the nitrobenzene cation with a geometry-dependent oscillator strength, reaching a maximum at 90° C-C-N-O dihedral angle with a corresponding energy gap of ∼2 eV. These results are consistent with the experimental observation in the nitrobenzene cation of a coherent superposition of vibrational states: a vibrational wave packet. Time-resolved measurements using a probe wavelength of 650 nm, nearly resonant with the strong transition, result in enhanced ion yield oscillation amplitudes as compared to excitation with the nonresonant 800 nm wavelength. Analogous behavior is found for the closely related molecules 2- and 4-nitrotoluene. These results demonstrate that computational chemistry can predict the best choice of probe wavelength in time-resolved measurements of vibrational coherent states in molecular cations.

Intersystem crossing of 2-Methlypyrazine studied by femtosecond photoelectron imaging

The ultrafast nonadibatic relaxation dynamics of the excited state of 2-methylpyrazine has been studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S&lt;sub&gt;1&lt;/sub&gt; of 2-methylpyrazine was excited by 323 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The lifetime of S&lt;sub&gt;1&lt;/sub&gt; state 98 ps is obtained by time-resolved mass spectroscopy. The intersystem crossing from the S&lt;sub&gt;1&lt;/sub&gt; state to the T&lt;sub&gt;1&lt;/sub&gt; state is observed on real time. The relaxation dynamics of S&lt;sub&gt;1&lt;/sub&gt; state of 2-methlypyrazine is different from that of pyrazine, the results show that the intersystem crossing process between S&lt;sub&gt;1&lt;/sub&gt; and T&lt;sub&gt;1&lt;/sub&gt; is the main relaxation channel of S&lt;sub&gt;1&lt;/sub&gt; state of 2-methlypyrazine, but the internal conversion process between S&lt;sub&gt;1&lt;/sub&gt; and S&lt;sub&gt;0&lt;/sub&gt; is also a main relaxation channel of S&lt;sub&gt;1&lt;/sub&gt; state. By using the advantages of femtosecond time-resolved photoelectron imaging, the photoelectron angular distribution at different pump-probe time delay was obtained experimentally. From the photoelectron angle distribution combined with photoelectron kinetic energy distributions, we tried to observe the field-free nonadiabatic alignment. However, due to the fact that the molecular symmetry of 2-methylpyrazine is lower than that of pyrazine, it is more challenging to observe the phenomenon of molecular nonadiabatic alignment with lower symmetry. Therefore, it is fail to observe nonadiabatic alignment feature of 2-methylpyrazine in this experiment. This work provides a clearer physical picture for S&lt;sub&gt;1&lt;/sub&gt; state nonadibatic relaxation dynamics of 2-methylpyrazine.

Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations.

The ultrafast dynamics of polyatomic radical cations contribute to important processes including energy transfer in photovoltaics, electron transfer in photocatalysis, radiation-induced DNA damage, and chemical reactions in the upper atmosphere and space. Probing these dynamics in the gas phase is challenging due to the rapid dissociation of polyatomic radical cations following electron removal, which arises from excess electronic excitation of the molecule during the ionization process. This Concept article introduces the reader to how the pump-probe technique of femtosecond time-resolved mass spectrometry (FTRMS) can overcome this challenge to capture coherent vibrational dynamics on the femtosecond timescale in polyatomic radical cations and enable the analysis of their dissociation pathways. Examples of FTRMS applied to three families of polyatomic radical cations are discussed.

Probing Coherent Vibrations of Organic Phosphonate Radical Cations with Femtosecond Time-Resolved Mass Spectrometry.

Organic phosphates and phosphonates are present in a number of cellular components that can be damaged by exposure to ionizing radiation. This work reports femtosecond time-resolved mass spectrometry (FTRMS) studies of three organic phosphonate radical cations that model the DNA sugar-phosphate backbone: dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), and diisopropyl methylphosphonate (DIMP). Upon ionization, each molecular radical cation exhibits unique oscillatory dynamics in its ion yields resulting from coherent vibrational excitation. DMMP has particularly well-resolved 45 fs ( cm) oscillations with a weak feature at 610–650 cm, while DIMP exhibits bimodal oscillations with a period of ∼55 fs and two frequency features at and 670–720 cm. In contrast, the oscillations in DEMP decay too rapidly for effective resolution. The low- and high-frequency oscillations in DMMP and DIMP are assigned to coherent excitation of the symmetric O–P–O bend and P–C stretch, respectively. The observation of the same ionization-induced coherently excited vibrations in related molecules suggests a possible common excitation pathway in ionized organophosphorus compounds of biological relevance, while the distinct oscillatory dynamics in each molecule points to the potential use of FTRMS to distinguish among fragment ions produced by related molecules.

How the methyl group position influences the ultrafast deactivation in aromatic radicals.

Excited xylyl (methyl-benzyl) radical isomers have been studied by femtosecond time-resolved photoelectron spectroscopy and mass spectrometry. Depending on the substitution we find different deactivation channels after excitation into the D3(2A'') state (310 nm, 4 eV). While the ortho and para isomer exhibit deactivation rates similar to the benzyl radical, meta-xylyl sticks out and depletes twice as fast into the vibrationally hot ground state. We found that a ring deformation mode rather than the methyl pseudorotation enables access to a conical intersection, which is responsible for the faster deactivation. Transitions in the photoelectron spectrum can be assigned to several Rydberg series with mostly d angular momentum components. Absorption of two 4 eV photons triggers hydrogen loss reactions on a femtosecond timescale.

Excited state dynamics of molecules studied with femtosecond time-resolved mass spectrometry and photoelectron imaging

Study of quantum states of molecules, especially the evolution of excited states can help to understand their basic features and the interactions among different states. Furthermore, the information about the chemical reaction process and the interactions among several reaction channels can be obtained. Femtosecond time-resolved mass spectrometry (TRMS) and time-resolved photoelectron imaging (TRPEI), which combine pump-probe technique with time of flight mass spectrometry and photoelectron imaging, are powerful tools for detecting the molecular quantum state and for studying the molecular quantum state interaction and molecular ultrafast dynamics. With these methods, the photochemistry and photophysics mechanism of isolated molecule reaction process can be investigated on a femtosecond time scale. The principles of TRMS and TRPEI are introduced here in detail. On the basis of substantial research achievements in our group, the applications of TRMS and TRPEI are presented in the study of ultrafast internal conversion and intersystem crossing, wavepacket evolution dynamics at excited states of polyatomic molecules, energy transfer process of polyatomic molecules, ultrafast photodissociation dynamics and structural evolution dynamics of molecular excited states. In the study of ultrafast internal conversion and intersystem crossing, the methyl substituted benzene derivatives and benzene halides are discussed as typical molecular systems. In the study of wavepacket evolution dynamics at excited states of polyatomic molecules, the real-time visualization of the dynamic evolution of CS2 4d and 6s Rydberg wave packet components, the vibrational wave packet dynamics in electronically excited pyrimidine, the rotational wave packet revivals and field-free alignment in excited o-dichlorobenzene are reported. In order to discuss the energy transfer process of polyatomic molecules, the intramolecular vibrational energy redisctribution between different vibrational states in p-difluorobenzene in the S1 low-energy regime and the intramolecular energy transfer between different electronic states in excited cyclopentanone are presented. For the study of ultrafast photodissociation dynamics, the dissociation constants and dynamics of the A band and even higher Rydberg states are investigated for the iodine alkanes and iodine cycloalkanes. Structural evolution dynamics of molecular excited states is the main focus of our recent research. The structural evolution dynamics can be extracted from the coherent superposition preparation of quantum states and the observation of quantum beat phenomenon, by taking 2, 4-difluorophenol and o-fluorophenol as examples. Time-dependent photoelectron peaks originating from the planar and nonplanar geometries in the first excited state in 2, 4-difluorophenol exhibit the clear beats with similar periodicities but a phase shift of π rad, offering an unambiguous picture of the oscillating nuclear motion between the planar geometry and the nonplanar minimum. Also, the structural evolution dynamics in o-fluorophenol via the butterfly vibration between planar geometry and nonplanar minimum is mapped directly. Finally, the potential developments and further possible research work and future directions of these techniques and researches are prospected.

Ultrafast α -CC bond cleavage of acetone upon excitation to 3p and 3d Rydberg states by femtosecond time-resolved photoelectron imaging.

The radiationless electronic relaxation and α -CC bond fission dynamics of jet-cooled acetone in the S1 (nπ*) state and in high-lying 3p and 3d Rydberg states have been investigated by femtosecond time-resolved mass spectrometry and photoelectron imaging. The S1 state was accessed by absorption of a UV pump photon at selected wavelengths between λ = 320 and 250 nm. The observed acetone mass signals and the S1 photoelectron band decayed on sub-picosecond time scales, consistent with a recently proposed ultrafast structural relaxation of the molecules in the S1 state away from the Franck-Condon probe window. No direct signatures could be observed by the experiments for CC dissociation on the S1 potential energy hypersurface in up to 1 ns. The observed acetyl mass signals at all pump wavelengths turned out to be associated with absorption by the molecules of one or more additional pump and/or probe photons. In particular, absorption of a second UV pump photon by the S1 (nπ*) state was found to populate a series of high-lying states belonging to the n = 3 Rydberg manifold. The respective transitions are favored by much larger cross sections compared to the S1 ← S0 transition. The characteristic energies revealed by the photoelectron images allowed for assignments to the 3p and 3dyz states. At two-photon excitation energies higher than 8.1 eV, an ultrafast reaction pathway for breaking the α -CC bond in 50-90 fs via the 3dyz Rydberg state and the elusive ππ* state was observed, explaining the formation of acetyl radicals after femtosecond laser excitation of acetone at these wavelengths.

Photodissociation Dynamics of 2-Iodotoluene Investigated by Femtosecond Time-Resolved Mass Spectrometry

The photodissociation dynamics of 2-iodotoluene following excitation at 266 nm have been investigated employing femtosecond time-resolved mass spectrometry. The photofragments are detected by multiphoton ionization using an intense laser field centered at 800 nm. A dissociation time of 380±50 fs was measured from the rising time of the co-fragments of toluene radical (C7H7) and iodine atom (I), which is attributed to the averaged time needed for the C−I bond breaking for the simultaneously excited nσ* and ππ* states by 266 nm pump light. In addition, a probe light centered at 298.23 nm corresponding to resonance wavelength of ground-state iodine atom is used to selectively ionize ground-state iodine atoms generated from the dissociation of initially populated nσ* and ππ* states. And a rise time of 400±50 fs is extracted from the fitting of time-dependent I+ transient, which is in agreement with the dissociation time obtained by multiphoton ionization with 800 nm, suggesting that the main dissociative products are ground-state iodine atoms.

The involvement of triplet receiver states in the ultrafast excited state processes of small esters.

The photoinduced processes of methyl formate and methyl acetate have been probed by femtosecond time-resolved mass spectrometry and photoelectron spectroscopy experiments supported by quantum chemical calculations. Upon excitation to a vibrationally hot S1 state both molecules were found to relax away from the Franck-Condon geometry faster than the cross-correlation (≈170 fs). During relaxation of the S1 surface intersystem crossing to the triplet manifold is possible via the T2 state which enables an El-Sayed allowed transition. The time-resolved photoelectron spectra show indications of a triplet state formed on an ultrafast timescale. We suggest that the ultrafast timescale of the intersystem crossing process is possible due to the shape of the potential energy surface directing and restricting the dynamics along the carbonyl group de-planarization coordinate, where an El-Sayed allowed intersystem crossing is possible at all times, and where the S1 and T2 states are nearly iso-energetic all the way along the reaction coordinate.

Photodissociation dynamics of allyl chloride at 200 and 266 nm studied by time-resolved mass spectrometry and photoelectron imaging.

The photodissociation dynamics of allyl chloride at 200 and 266 nm has been studied by femtosecond time-resolved mass spectrometry coupled with photoelectron imaging. The molecule was prepared to different excited states by selectively pumping with 400 or 266 nm pulse. The dissociated products were then probed by multiphoton ionization with 800 nm pulse. After absorbing two photons at 400 nm, several dissociation channels were directly observed from the mass spectrum. The two important channels, C-Cl fission and HCl elimination, were found to decay with multiexponential functions. For C-Cl fission, two time constants, 48 ± 1 fs and 85 ± 40 ps, were observed. The first one was due to the fast predissociation process on the repulsive nσ*/πσ* state. The second one could be ascribed to dissociation on the vibrationally excited ground state which is generated after internal conversion from the initially prepared ππ* state. HCl elimination, which is a typical example of a molecular elimination reaction, was found to proceed with two time constants, 600 ± 135 fs and 14 ± 2 ps. We assigned the first one to dissociation on the excited state and the second one to the internal conversion from the ππ* state to the ground state and then dissociation on the ground state. As we excited the molecule with 266 nm light, the transient signals decayed exponentially with a time constant of ∼48 fs, which is coincident with the time scale of C-halogen direct dissociation. Photoelectron images, which provided translational and angular distributions of the generated electron, were also recorded. Detailed analysis of the kinetic energy distribution strongly suggested that C3H4(+) and C3H5(+) were generated from ionization of the neutral radical. The present study reveals the dissociation dynamics of allyl chloride in a time-resolved way.

Internal conversion mediated by specific nuclear motions: The nitrogen inversion in amines

To investigate the influence of amine structure on the internal conversion from S2 to S1, three aliphatic amines containing the same number of degrees of freedom, but with different degree of N-substitution, were investigated with femtosecond time-resolved mass spectrometry. As N-substituents lower the excitation energies, and the excitation in all cases is by a 200nm photon, the S1 density-of-states in the Franck–Condon region will be high for the more N-alkylated amine. This, according to standard models, should lead to faster internal conversion. The experimental results are in contrast to this, indicating non-ergodic behavior and a coupling of the states by very few molecular vibrations. This was further investigated by substituting differing numbers of hydrogen atoms with deuterium in two amines, one primary and one tertiary, thus freezing the degrees of freedom while altering the N-substituent mass. In both case, deuteration resulted in a significantly longer lifetime of the initially excited state, confirming the importance of the N-inversion motion in the description of the dynamics of internal conversion.

Desorption Dynamics of Tetracene Ion from Tetracene-Doped Anthracene Crystals Studied by Femtosecond Time-Resolved Mass Spectrometry

The desorption dynamics of tetracene ion ([tetracene]+) from tetracene-doped anthracene (TDA) crystals in which efficient energy transfer from anthracene to tetracene occurs was investigated by means of femtosecond time-resolved mass spectrometry. Using a 400 nm pump pulse, anthracene molecules in TDA crystals are initially excited to the S1 state. After a certain delay (<10 ps) of pumping, the excitation energy in the S1 anthracene is transferred to a tetracene molecule to produce the S1 state. Tetracene in the S1 state is then ionized by a time-delayed probe pulse (266 nm) and desorbed tetracene ions from the crystals are monitored with a time-of-flight mass spectrometer. On the time-resolved mass spectra, the peak intensity of the tetracene ions shows a gradual increase with increasing delay time. The rise time constant of tetracene ion, which is considered to the desorption time from the crystals, was determined to be 80.8 and 94.7 ps for TDA crystals with anthracene/tetracene concentration ratios of ...