Browsing by Author "Ni, Chi-Kung"

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Advantage of spatial map ion imaging in the study of large molecule photodissociation
(2017-07-07)
Lee, Chin
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Lin, Yen-Cheng
;
Lee, Shih-Huang
;
Lee, Yin-Yu
;
Tseng, Chien-Ming
The original ion imaging technique has low velocity resolution, and currently, photodissociation is mostly investigated using velocity map ion imaging. However, separating signals from the background (resulting from undissociated excited parent molecules) is difficult when velocity map ion imaging is used for the photodissociation of large molecules (number of atoms >= 10). In this study, we used the photodissociation of phenol at the S-1 band origin as an example to demonstrate how our multimass ion imaging technique, based on modified spatial map ion imaging, can overcome this difficulty. The photofragment translational energy distribution obtained when multimass ion imaging was used differed considerably from that obtained when velocity map ion imaging and Rydberg atom tagging were used. We used conventional translational spectroscopy as a second method to further confirm the experimental results, and we conclude that data should be interpreted carefully when velocity map ion imaging or Rydberg atom tagging is used in the photodissociation of large molecules. Finally, we propose a modified velocity map ion imaging technique without the disadvantages of the current velocity map ion imaging technique. Published by AIP Publishing.
Core Excitation, Specific Dissociation, and the Effect of the Size of Aromatic Molecules Connected to Oxygen: Phenyl Ether and 1,3-Diphenoxybenzene
(2014-09-11)
Lin, Yi-Shiue
;
Lin, Shu-Yu
;
Lee, Yuan T.
;
Tseng, Chien-Ming
;
Ni, Chi-Kung
Near-edge X-ray absorption fine structure (NEXAFS) spectra of phenyl ether at the carbon K-edge and 1,3-diphenoxybenzene at both the carbon and oxygen K-edges were measured in the total ion yield mode using X-rays from a synchrotron and a reflectron time-of-flight mass spectrometer. Time-dependent density functional theory was adopted to calculate the carbon and oxygen K-edge NEXAFS spectra of phenol, phenyl ether, and 1,3-diphenoxybenzene. The assignments and a comparison of the experimental and calculated spectra are presented. The mass spectra of ionic products formed after X-ray absorption at various excitation energies are also reported. Specific dissociations were observed for the 1s --> pi* transition of phenyl ether. In comparison with phenol and phenyl ether, the dependence of the fragmentation on the excitation site and destination state was weak in 1,3-diphenoxybenzene, likely as a result of delocalization of the valence electrons and rapid randomization of energy.
Excited-state dissociation dynamics of phenol studied by a new time-resolved technique
(2018-02-21)
Lin, Yen-Cheng
;
Lee, Chin
;
Lee, Shih-Huang
;
Lee, Yin-Yu
;
Lee, Yuan T.
Phenol is an important model molecule for the theoretical and experimental investigation of dissociation in the multistate potential energy surfaces. Recent theoretical calculations [X. Xu et al., J. Am. Chem. Soc. 136, 16378 (2014)] suggest that the phenoxyl radical produced in both the X and A states from the O-H bond fission in phenol can contribute substantially to the slow component of photofragment translational energy distribution. However, current experimental techniques struggle to separate the contributions from different dissociation pathways. A new type of time-resolved pump-probe experiment is described that enables the selection of the products generated from a specific time window after molecules are excited by a pump laser pulse and can quantitatively characterize the translational energy distribution and branching ratio of each dissociation pathway. This method modifies conventional photofragment translational spectroscopy by reducing the acceptance angles of the detection region and changing the interaction region of the pump laser beam and the molecular beam along the molecular beam axis. The translational energy distributions and branching ratios of the phenoxyl radicals produced in the X, A, and B states from the photodissociation of phenol at 213 and 193 nm are reported. Unlike other techniques, this method has no interference from the undissociated hot molecules. It can ultimately become a standard pump-probe technique for the study of large molecule photodissociation in multistates. Published by AIP Publishing.
Fluorescence spectroscopy of UV-MALDI matrices and implications of ionization mechanisms
(2014-10-28)
Lin, Hou-Yu
;
Hsu, Hsu Chen
;
Lu, I-Chung
;
Hsu, Kuo-Tung
;
Liao, Chih-Yu
Matrix-assisted laser desorption ionization (MALDI) has been widely used in the mass analysis of biomolecules; however, there are a lot of debates about the ionization mechanisms. Previous studies have indicated that S-1-S-1 annihilation might be a key process in the generation of primary ions. This study investigates S-1-S-1 annihilation by examining the time-resolved fluorescence spectra of 12 matrices. No S-1-S-1 annihilation was observed in six of these matrices (3-hydroxy-picolinic acid, 6-aza-2-thiothymine, 2,4-dihydroxy-acetophenone, 2,6-dihydroxy-acetophenone, 2,4,6-trihydroxy-acetophenone, and ferulic acid). We observed two matrix molecules reacting in an electronically excited state (S-1) in five of these matrices (2,5-dihydroxybenzoic acid, alpha-cyano-4-hydroxycinnamic acid, 2,5-dihydroxy-acetophenone, 2,3-dihydroxybenzoic acid, and 2,6-dihydroxybenzoic acid), and S-1-S-1 annihilation was a possible reaction. Among these five matrices, no S-1-S-1 annihilation was observed for 2,3-dihydroxybenzoic acid in typical peak power region of nanosecond laser pulses in MALDI, but a very small value of reaction rate constant was observed only in the high peak power region. The excited-state lifetime of sinapinic acid was too short to determine whether the molecules reacted in an electronically excited state. No correlation was observed between the ion generation efficiency of MALDI and S-1-S-1 annihilation. The results indicate that the proposal of S-1-S-1 annihilation is unnecessary in MALDI and energy pooling model for MALDI ionization mechanism has to be modified. (C) 2014 AIP Publishing LLC.
Highly Selective Dissociation of a Peptide Bond Following Excitation of Core Electrons
(2015-06-18)
Lin, Yi-Shiue
;
Tsai, Cheng-Cheng
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Lin, Huei-Ru
;
Hsieh, Tsung-Lin
;
Chen, Jien-Lian
The controlled breaking of a specific chemical bond with photons in complex molecules remains a major challenge in chemistry. In principle, using the K-edge absorption of a particular atomic element, one might excite selectively a specific atomic entity in a molecule. We report here highly selective dissociation of the peptide bonds in N-methylformamide and N-methylacetamide on tuning the X-ray wavelength to the K-edge absorption of the atoms connected to (or near) the peptide bond. The high selectivity (56-71%) of this cleavage arises from the large energy shift of X-ray absorption, a large overlap of the Is orbital and the valence pi* orbital that is highly localized on a peptide bond with antibonding character, and the relatively low bond energy of the peptide bonds. These characteristics indicate that the high selectivity on bond dissociation following core excitation could be a general feature for molecules containing peptide bonds.
Ion Intensity and Thermal Proton Transfer in Ultraviolet Matrix-Assisted Laser Desorption/Ionization
(2014-04-17)
Lu, I-Chung
;
Lee, Chuping
;
Chen, Hui-Yuan
;
Lin, Hou-Yu
;
Hung, Sheng-Wei
The ionization mechanism of ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI) was investigated by measuring the total cation intensity (not including sodiated and potasiated ions) as a function of analyte concentration (arginine, histidine, and glycine) in a matrix of 2,4,6-trihydroxyacetophenone (THAP). The total ion intensity increased up to SS times near the laser fluence threshold as the arginine concentration increased from 0% to 1%. The increases were small for histidine, and a minimal increase occurred for glycine. Time-resolved fluorescence intensity was employed to investigate how analytes affected the energy pooling of the matrix. No detectable energy pooling was observed for pure THAP and THAP/analyte mixtures. The results can be described by using a thermal proton transfer model, which suggested that thermally induced proton transfer is crucial in the primary ion generation in UV-MALDI.
Is energy pooling necessary in ultraviolet matrix-assisted laser desorption/ionization?
(2014-01-15)
Lin, Hou-Yu
;
Song, Botao
;
Lu, I-Chung
;
Hsu, Kuo-Tung
;
Liao, Chih-Yu
RATIONALEEnergy pooling has been suggested as the key process for generating the primary ions during ultraviolet matrix-assisted laser desorption/ionization (UV-MALDI). In previous studies, decreases in fluorescence quantum yields as laser fluence increased for 2-aminobenzoic acid, 2,5-dihydroxybenzoic acid (2,5-DHB), and 3-hydroxypicolinic acid were used as evidence of energy pooling. This work extends the research to other matrices and addresses whether energy pooling is a universal property in UV-MALDI. METHODSEnergy pooling was investigated in a time-resolved fluorescence experiment by using a short laser pulse (355nm, 20ps pulse width) for excitation and a streak camera (1ps time resolution) for fluorescence detection. RESULTSThe excited-state lifetime of 2,5-DHB decreased with increases in laser fluence. This suggests that a reaction occurs between two excited molecules, and that energy pooling may be one of the possible reactions. However, the excited-state lifetime of 2,4,6-trihydroxyacetophenone (THAP) did not change with increases in laser fluence. The upper limit of the energy pooling rate constant for THAP is estimated to be approximately 100-500 times smaller than that of 2,5-DHB. CONCLUSIONSThe small energy pooling rate constant for THAP indicates that the potential contribution of the energy pooling mechanism to the generation of THAP matrix primary ions should be reconsidered. Copyright (c) 2013 John Wiley & Sons, Ltd.
Measurement and prediction of the NEXAFS spectra of pyrimidine and purine and the dissociation following the core excitation
(2015-09-01)
Lin, Yi-Shiue
;
Lin, Huei-Ru
;
Liu, Wei-Lun
;
Lee, Yuan T.
;
Tseng, Chien-Ming
Near-edge X-ray absorption fine structure spectra of pyrimidine and purine at the carbon and nitrogen K-edges were measured using synchrotron radiation and a reflectron time-of-flight mass spectrometer. Spectrum assignments based on the time-dependent density functional theory calculation were presented. From the comparison of the experimental and theoretical results, the nature of various core-excitation transitions and the chemical shifts in the absorption energies were elucidated. The dissociation fragments following the core excitation were analyzed, and the enhancements on the fragment branching ratios were observed. The possible identities and dissociation mechanisms of the major fragments were discussed. (C) 2015 Elsevier By. All rights reserved .
Near-Edge X-ray Absorption Fine Structure Spectra and Site-Selective Dissociation of Phenol
(2014-03-06)
Lin, Yi-Shiue
;
Lu, Kun-Ta
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Lee, Yuan T.
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Tseng, Chien-Ming
;
Ni, Chi-Kung
A time-of-flight mass spectrometer with orthogonal acceleration and soft X-rays from synchrotron radiation were utilized to measure near-edge X-ray absorption fine structure (NEXAFS) spectra of carbon and oxygen in phenol and the corresponding ionic fragments following core excitation. The photon energies were in the range of 284-298 eV for the carbon K-edge and 529.5-554.5 eV for the oxygen K-edge. The total ion yield, ion intensity for each ionic fragment, and ion intensity ratio, defined as ion intensity divided by total ion yield, were measured as a function of photon energy. Possible mechanisms of dissociation are proposed and enhancements of specific products of dissociation are reported. In general, the enhancement of these specific products is small in the carbon K-edge region but is clear for some products at the oxygen K-edge. In particular, elimination of the H atom from the hydroxyl group was observed only at the oxygen K-edge. One remarkable result is that an excitation of a core-level electron of oxygen greatly enhanced the cleavage of specific C-C bonds.
NEXAFS spectra and specific dissociation of oligo-peptide model molecules
(2019-08-01)
Chiang, Yu-Ju
;
Huang, Wan-Chou
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Ni, Chi-Kung
;
Liu, Chen-Lin
;
Tsai, Cheng-Cheng
Total-ion-yield near-edge X-ray absorption fine-structure (NEXAFS) spectra of three oligo-peptide model molecules were recorded. The NEXAFS spectra were also calculated with time-dependent density functional theory; the calculated spectra are consistent with the experimental measurements. The ionic products from core-excited molecules and their branching ratios as functions of excitation energy show branching ratios 42%-75% of dissociation of the peptide bond, especially at resonant excitations. The major destination orbitals assigned at these excitation energies show significant antibonding character along peptide bonds, which causes large probabilities of bond cleavage. (C) 2019 Author(s).
Photodissociation Dynamics of Benzaldehyde (C(6)H(5)CHO) at 266, 248, and 193 nm
(2011-11-04)
Bagchi, Arnab
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Huang, Yu-Hsuan
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Xu, Z. F.
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Raghunath, P.
;
Lee, Yuan T.
The photodissociation of gaseous benzaldehyde (C(6)H(5)CHO) at 193, 248, and 266 nm using multimass ion imaging and step-scan time-resolved Fourier-transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6-311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C(6)H(5)+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol(-1) dominates. The C(6)H(6)+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C(6)H(5)CHO, nu(3) (nu = 1) of HCO, and levels nu <= 2, J <= 43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol(-1) and vibrational energy of approximately 6 kJ mol(-1). Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C(6)H(5)+HCO channel becomes the only major channel with a branching ratio of 0.82 +/- 0.10 and an increased proportion of the slow component; IR emission from levels nu(1) (nu = 1) and nu(3) (nu = 1 and 2) of HCO and nu <= 2, J <= 43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical-theory predicted results based on the computed singlet and triplet potential-energy surfaces.
Photodissociation Dynamics of Benzaldehyde (C6H5CHO) at 266, 248, and 193 nm
(2011-11-04)
Bagchi, Arnab
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Huang, Yu-Hsuan
;
Xu, Z. F.
;
Raghunath, P.
;
Lee, Yuan T.
The photodissociation of gaseous benzaldehyde (C6H5CHO) at 193, 248, and 266 nm using multimass ion imaging and step-scan time-resolved Fourier-transform infrared emission techniques is investigated. We also characterize the potential energies with the CCSD(T)/6-311+G(3df,2p) method and predict the branching ratios for various channels of dissociation. Upon photolysis at 248 and 266 nm, two major channels for formation of HCO and CO, with relative branching of 0.37:0.63 and 0.20:0.80, respectively, are observed. The C6H5+HCO channel has two components with large and small recoil velocities; the rapid component with average translational energy of approximately 25 kJ mol(-1) dominates. The C6H6+CO channel has a similar distribution of translational energy for these two components. IR emission from internally excited C6H5CHO, nu(3) (nu = 1) of HCO, and levels nu <= 2, J <= 43 of CO are observed; the latter has an average rotational energy of approximately 13 kJ mol(-1) and vibrational energy of approximately 6 kJ mol(-1). Upon photolysis at 193 nm, similar distributions of energy are observed, except that the C6H5+HCO channel becomes the only major channel with a branching ratio of 0.82 +/- 0.10 and an increased proportion of the slow component; IR emission from levels nu(1) (nu = 1) and nu(3) (nu = 1 and 2) of HCO and nu <= 2, J <= 43 of CO are observed; the latter has an average energy similar to that observed in photolysis at 248 nm. The observed product yields at different dissociation energies are compared to statistical-theory predicted results based on the computed singlet and triplet potential-energy surfaces.
Photodissociation dynamics of nitrobenzene and o-nitrotoluene
(2007-02-14)
Lin, Ming-Fu
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Lee, Yuan T.
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Ni, Chi-Kung
;
Xu, Shucheng
;
Lin, M. C.
Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO2 elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO2 elimination channels were determined to be NO/NO2=0.32 +/- 0.12 (193 nm), 0.26 +/- 0.12 (248 nm), and 0.4 +/- 0.12(266 nm) for nitrobenzene and 0.42 +/- 0.12(193 nm) and 0.3 +/- 0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results. (c) 2007 American Institute of Physics.
Photodissociation dynamics of nitrobenzene and o-nitrotoluene
(2007-02-14)
Lin, Ming-Fu
;
Lee, Yuan T.
;
Ni, Chi-Kung
;
Xu, Shucheng
;
Lin, M. C.
Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO2 elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO2 elimination channels were determined to be NO/NO2=0.32 +/- 0.12 (193 nm), 0.26 +/- 0.12 (248 nm), and 0.4 +/- 0.12(266 nm) for nitrobenzene and 0.42 +/- 0.12(193 nm) and 0.3 +/- 0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results. (c) 2007 American Institute of Physics.
Photodissociation dynamics of phenol
(2007-09-27)
Tseng, Chien-Ming
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Lee, Yuan T.
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Lin, Ming-Fu
;
Ni, Chi-Kung
;
Liu, Suet-Yi
The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H2O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or HO occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1 <= v <= 4) in the spectral region of 1860-2330 cm(-1) were detected upon photolysis at 193 nm. After a correction for rotational quenching, CO (v 4) shows a nascent rotational temperature of similar to 4600 K. The observed vibrational distribution of (v = 1)1(v 2)/(v = 3)/(v = 4) = 64.3/22.2/9.1/4.4 corresponds to a vibrational temperature of 3350 +/- 20 K. An average rotational energy of 6.9 +/- 0.7 kcal mol(-1) and vibrational energy of 3.8 +/- 0.7 kcal mol(-1) are observed for the CO product. The dissociation channels, translational energy distributions of the photofragment, and vibrational and rotational energies of product CO are consistent with potential energy surfaces from quantum chemical calculations and the branching ratios from an RRKM calculation.
Rotationally resolved ultrahigh-resolution laser spectroscopy of the S(2) (1)A(1)<- S(0) (1)A(1) transition of azulene
(2009-07-14)
Semba, Yosuke
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Yoshida, Kazuto
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Kasahara, Shunji
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Ni, Chi-Kung
;
Hsu, Yen-Chu
We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S(2) (1)A(1)<- S(0) (1)A(1) transition of jet-cooled azulene. The observed linewidth is 0.0017 cm(-1), which corresponds to the lifetime of 3.1 ns in the S(2) state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S(0) and S(2) states (C(2 upsilon) symmetry). Rotational constants of the S(2) state are almost identical to those of the S(0) state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S(2) (1)A(1) state of azulene was identified to be IC to the S(1) (1)B(2) state. However, this S(2)-> S(1) IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S(1) (1)B(2) state, which is also responsible for the extraordinarily fast S(1)-> S(0) IC in the isolated azulene molecule. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3168394]
Rotationally resolved ultrahigh-resolution laser spectroscopy of the S-2 (1)A(1)<- S-0 (1)A(1) transition of azulene
(2009-07-14)
Semba, Yosuke
;
Yoshida, Kazuto
;
Kasahara, Shunji
;
Ni, Chi-Kung
;
Hsu, Yen-Chu
We have observed rotationally resolved ultrahigh-resolution fluorescence excitation spectra of the 0(0)(0) (a-type) and 0(0)(0)+467 cm(-1) (b-type) bands of the S-2 (1)A(1)<- S-0 (1)A(1) transition of jet-cooled azulene. The observed linewidth is 0.0017 cm(-1), which corresponds to the lifetime of 3.1 ns in the S-2 state. Zeeman splitting of rotational lines is very small so that intersystem crossing to the triplet state is considered to be very slow. Inertial defect is very small and the molecule is considered to be planar in the S-0 and S-2 states (C-2 upsilon symmetry). Rotational constants of the S-2 state are almost identical to those of the S-0 state, indicating that geometrical structure is similar in both electronic states. In this case, internal conversion (IC) by vibronic coupling is thought to be inactive. Therefore, the main radiationless transition process in the S-2 (1)A(1) state of azulene was identified to be IC to the S-1 B-1(2) state. However, this S-2 -> S-1 IC is still slower than that of conventional polycyclic aromatic hydrocarbons. We consider it to be due to the shallower potential energy curve in the S-1 B-1(2) state, which is also responsible for the extraordinarily fast S-1 -> S-0 IC in the isolated azulene molecule. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3168394]
Selectivity of peptide bond dissociation on excitation of a core electron: Effects of a phenyl group
(2016-09-01)
Tsai, Cheng-Cheng
;
Chen, Jien-Lian
;
Hu, Wei-Ping
;
Lin, Yi-Shiue
;
Lin, Huei-Ru
The selective dissociation of a peptide bond upon excitation of a core electron in acetanilide and N-benzylacetamide was investigated. The total-ion-yield near-edge X-ray absorption fine structure spectra were recorded and compared with the predictions from time-dependent density functional theory. The branching ratios for the dissociation of a peptide bond are observed as 16-34% which is quite significant. This study explores the core-excitation, the X-ray photodissociation pathways, and the theoretical explanation of the NEXAFS spectra of organic molecules containing both a peptide bond and a phenyl group. (C) 2016 Elsevier B.V. All rights reserved.
Triplet vs pi sigma* state mediated N-H dissociation of aniline
(2019-10-14)
Jhang, Wan Ru
;
Lai, Hsin Ying
;
Lin, Yen-Cheng
;
Lee, Chin
;
Lee, Shih-Huang
UV-excited aromatic molecules with N-H/O-H moieties often possess an important nonradiative relaxation pathway, from an optically bright pi pi* state to a dark dissociative pi sigma* state. We apply a new time-selected photofragment translational spectroscopy method to disclose a previously unknown triplet-mediated N-H dissociation of aniline prevented by the multiphoton dissociative ionization in conventional methods. We further determined the branching fractions of aniline dissociated in the pi sigma*, triplet, and ground states at 248 nm. Additionally, we selectively captured the population changes in the singlet and triplet states with ionization from different laser wavelengths, 355 or 266 nm, in time-resolved photoion yields. The combination of experimental data enables us to uniquely determine the relative ionization cross sections of the singlet and triplet states at an ionization laser wavelength of 266 nm and allows us to extensively measure the rate constants of intersystem crossing and the branching fractions at various excitation wavelengths. Published under license by AIP Publishing.