Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene

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Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene on the C Band Chunlong Hu, Ying Tang, Xinli Song, Zhiming Liu, and Bing Zhang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b09610 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 11, 2016

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The Journal of Physical Chemistry

Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene on the C Band Chunlong Hu†,‡, Ying Tang*,†,‡, Xinli Song†,‡, Zhiming Liu†,‡, and Bing Zhang*,†,‡

†

State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics,

Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, P. R. China ‡

*

University of Chinese Academy of Sciences, Beijing 100049, P. R. China

Ying Tang, E-mail: [email protected]; *Bing Zhang, E-mail: [email protected]

Phone: +86-27-87197441. Fax: +86-27-87198491

*

To Whom correspondence should be addressed 1

ACS Paragon Plus Environment


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ABSTRACT The photodissociation dynamics of highly excited iodobenzene from the C band absorption has been studied by femtosecond time-resolved ion yields techniques. Detailed photodissociation routes are discussed with the aid of high-level, spin-orbit resolved ab initio calculations of one-dimensional potential energy curves. Upon 200 nm excitation within the C band, iodobenzene molecules on 7B2 and 7B1 states decay to 7A1 and 8B2 states through internal conversion in 75 fs, with electronic energy converted into high vibrational energy of 7A1 and 8B2 states. Subsequently, 7A1 and 8B2 states decay through internal vibrational energy redistribution in 540 fs, accompanied with the excited C-I mode and the resulting cleavage of the C-I bond. The overall time for the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(2P3/2) production is 1.2 ps.

1. INTRODUCTION Aryl iodides have been extensively investigated as models of large molecules displaying complex photodissociation dynamics,1-19 especially for iodobenzene (C6H5I, PhI), the simplest aryl iodide. The pioneering work on the ultra-violet spectrum of PhI by Kimura and co-workers has presented intense structures in the wavelength range of 155-235 nm, and a weaker A band feature at longer wavelengths extending to 330 nm.1 The absorption spectrum is complex and very different from those of benzene and other mono-substituted benzenes. Two types of transitions are accidentally superposed:2-5 the excitation of ring π electrons analogous to those in benzene and the excitation of n electrons similar to those in alkyl iodides. This makes PhI the interesting molecule whose photodissociation has received considerable attention. The photodissociation dynamics of PhI is very different from those of alkyl iodides. Alkyl iodides are known to photodissociate rapidly upon excitation to the A band continuum due to the σ*←n promotion.2 As for PhI, in addition to the (n,σ*) repulsive electronic states of the C-I bond found in alkyl iodides, the presence of 2


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benzene ring π electron system provides singlet and triplet (π,π*) bound electronic states which may lead to predissociation because of the mixing between these (π,π*) and (n,σ*) states.6 Through measurements of the angular distribution of iodine fragments, Bersohn’s group has found the anisotropy parameter β lower than those observed for alkyl iodides, suggesting the presence of a predissociation channel.7-8 With state-selective photofragment translational spectroscopy technique, El-Sayed’s group has determined the spatial and velocity distributions of the ground state nascent iodine atoms and observed two different velocity distributions.2,9 The high and sharp velocity distribution with highly anisotropic spatial distribution was assigned to the rapid dissociation similar to alkyl iodides, the lower and broader velocity distribution with a lower spatial anisotropy was assigned to the predissociation from a benzene (π,π*) type triplet state.2,9 Further, Zewail’s group has reported the first time-resolved study of photodissociation of PhI.3,10 The dissociation of PhI was found to occur by two sub-picosecond pathways after 278 nm excitation: a direct-mode dissociation in 400 fs and a complex-mode dissociation in 600 fs arising from internal vibrational energy redistribution (IVR) in the benzene ring.3,10 Also, Davidsson and co-workers have performed time-resolved study of PhI photodissociation with 266 nm excitation, and found the kinetics of PhI can be well characterized by two time constants of 350 and 700 fs,11 consistent with Zewail’s results.3,10 As for the branching ratios of different dissociation channels, they were also experimentally determined with velocity map ion imaging technique.6,12 Thus far, the photodissociation dynamics of PhI has been well established following electronic excitation via the A continuum. However, photoinduced dynamics of PhI at high-lying states is still unclear, although these states contribute to the major absorption in the UV spectrum.1 Bersohn’s group has studied the photodissociation of PhI at 193 nm.7 Different from the dissociation in the A band, they found that the actual dissociation pathway at 193 nm depended on the competition between intersystem crossing (ISC) and internal conversion (IC), and PhI dissociated from S3, S2, and S1 states.7 Resonance Raman spectroscopy experiments performed by Kinsey and co-workers have provided insights into the initial dynamics 3



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of PhI following excitation in the region of 219-233 nm for the B band absorption.13 They revealed that the excitation was dominated by the benzene ring-based modes, which leads to the conclusion that PhI fragmentation involves a predissociation mechanism at these wavelengths. The photodissociation dynamics is more complex for PhI in high-lying states due to the complicated structures of potential energy surfaces and the existed multiple crossing of the excited states. As far as we know, there’s no time-resolved research determining the intermediate times which govern the predissociation mechanism for high-lying states of PhI. In the present work, we report a real-time investigation of photoinduced dynamics of PhI in high-lying states using time-resolved mass spectrometry. We also performed high-level, spin-orbit resolved ab initio calculation of the potential energy curves (PECs) for the ground and 31 excited states along the C-I dissociation coordinate, and detailed photodissociation routes are discussed with the aid of ab initio calculations.

2. METHODS 2.1. Experimental details. The experimental apparatus has been described in detail in previous work,20 and only the details relevant to the present experiments will be given here. The laser is a Coherent amplified Ti:Sapphire system delivering 4 mJ pulses of 100 fs duration at a 1 kHz repetition rate centered at 800 nm. The fundamental output was split with beam splitters, one arm of 1.5 mJ was used to generate a 200 nm pump beam with a three-stage sum-frequency mixing scheme, another arm of 1.5 mJ was used to pump an optical parametric amplifier (OPA) (Light Conversion, TOPAS). The signal output of TOPAS was frequency quadrupled to produce the probe beam of 298.2 nm. Considering the linewidth (full width at half maximum, FWHM) of the probe pulse is about 1.5 nm, the probe wavelength of 298.2 nm was chosen to exclusively detect the fragment of I(2P3/2) from photodissociation via resonant enhanced multiphoton ionization (REMPI) scheme. A retroreflector mounted on a computer-controlled, motorized delay stage in the probe arm provided controllable delay between the pump and probe pulses. The pump and probe beams were introduced into the ionization chamber collinearly through a dichroic mirror, and 4


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each was focused with a lens (f = 30 cm) into the interaction region and overlapped by a supersonic molecular beam. The cross-correlation was measured to be 200±20 fs by recording the difference frequency generation (DFG) signal in a very thin BBO crystal. PhI (99% purity, purchased from Aladdin Reagent) was used without further purification. It was carried by helium gas at about 1 atm backing pressure, and expanded through a pulsed valve to generate a pulsed molecular beam. The molecular beam was skimmed and introduced into the ionization chamber where it intersected with the laser beams perpendicularly in a two-stage ion lens region. In order to avoid clusters, we worked on the leading edge of the molecular beam. The pump laser pulse of 200 nm initiated the dissociation and the probe laser pulse of 298.2 nm monitored the following dynamics, the resulting ions were extracted by a set of electrostatic lenses and then detected by a chevron MCP arrangement. The ion signal was fed into a digital oscilloscope and recorded with a computer-based data acquisition system written with LabVIEW software. 2.2. Computational methods. Theoretical calculations of one-dimensional PECs were also performed to elucidate the photodissociation mechanism. The ground state geometry of PhI was first optimized in C2v symmetry at the CASPT2 level of theory,21 using cc-pVTZ basis set for C and H atoms,22 cc-pVTZ-pp for I atom.23 As for I atom, a 46 electron relativistic effective core potential (ECP) was used to describe spin-orbit coupling for valence electrons.24 The selected active space (12e, 10o) comprises 2a1, 5b1, 1b2 and 2a2 orbitals, which includes two lone pairs of Iodine, σ and σ* orbitals on the C-I bond, three phenyl (C6H5, Ph) π orbitals, and three phenyl π* orbitals. All calculations used the Molpro 2012 computational package.25 Starting from the optimized CASPT2 geometry, the potential energy curves in the RC-I coordinates were calculated for the ground (1A1), first two 1A1 and first three 1B1 states, 1B2, 1A2, 3A1, 3B1, 3B2 , 3A2 excited states with the coordinates of the Ph ring frozen at the ground state CASPT2 equilibrium geometry, using state-averaged (SA) CASSCF method.26 Separate state averaged calculations were done for each spin and symmetry case. CASPT2 spin-orbit free energies were then calculated for all the 5



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states at the same geometries using the SA-CASSCF as a reference wavefunction. For all CASPT2 calculations an imaginary level shift of 0.3 a.u. was required to avoid intrude state problems and the 1s carbon electrons were frozen. Spin-orbit coupled (SOC) states were calculated by evaluating Ĥso in a basis of ψelec.27 CASPT2 energies of the spin-orbit free (SOF) states were used in the diagonalization of the spin-orbit coupling matrix. The vertical excitation energy Tv and oscillation strength f values between the 32 SOC states along with the SOF states contributing to the SOC states were computed at each C-I internuclear distance.

3. RESULTS The photodissociation dynamics of PhI at 200 nm was studied by monitoring the decay of the parent ions and the formation of fragment ions produced by the pump and probe laser pulses. The pump pulse of 200 nm excites the PhI molecules and initiates the dissociation, leading to phenyl radicals and iodine atoms in its ground state I(2P3/2) and spin-orbit excited state I*(2P1/2) (hereafter referred to as I and I*, respectively). The probe pulse of 298.2 nm, which covers the resonance line of I, was used to ionize the parent PhI molecules and phenyl radicals and also detect the free iodine atoms of I(2P3/2) through (2+1) REMPI process. 3.1. Experimental results. Figure 1 shows the typical time-of-flight (TOF) mass spectra observed in our experiments with pump intensity at 0.4 µJ/pulse and probe intensity at 1.8 µJ/pulse. In order to avoid multiphoton ionization of PhI, the pulse energy of the pump beam was attenuated using a neutral density filter, and no ions were observed with only 200 nm pump beam radiation. Figure 1a shows the typical TOF mass spectra measured with only the radiation of 298.2 nm probe beam, in which the parent ion C6H5I+ (PhI+), the phenyl ion C6H5+ (Ph+) and the alkyl ion fragments C4Hn+ were observed. Considering the interaction time between the molecules and photons is very short under our ultrafast laser conditions, only the ladder mechanism is considered for one-color multiphoton ionization.28 It means that the fragment ions of Ph+ and C4Hn+ were produced from the fragmentation of the parent ion PhI+. As shown in Figure 1a, to a large extent the primary formed parent 6


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ions PhI+ underwent photo-induced fragmentation into phenyl ions Ph+ and alkyl ion fragments C4Hn+, and the intensities of Ph+ and C4Hn+ relative to the parent ions PhI+ increased with the increasing laser power of the probe beam, which further confirms the ladder mechanism. Note that no fragment ion of I+ was observed with 298.2 nm REMPI detection, this implies that the appearance energy of I+ from decomposition of PhI+ is too high to reach with our experimental condition, and the dissociation time for the generation of neutral I fragment is longer than the pulse width of the probe beam. Figure 1b and 1c shows the typical two-color mass spectra measured near zero-of-time and at an enough delay time of 8 ps, respectively. With the increase of the delay time, we can clearly see the increase of I+ and decrease of PhI+ correspondingly.

Figure 1. The TOF mass spectra of PhI were measured with (a) only probe pulse of 298.2 nm, (b) 200 nm pump pulse and 298.2 nm probe pulse near zero-of-time, (c) both the pump and probe beam at the delay time of ∆t (pump-probe) = 8 ps.

Figure 2 shows the time-resolved transients collected at parent PhI+ and fragment Ph+ ion channels following excitation of 200 nm pump laser and further photoionization with 298.2 nm probe laser pulses. The transient of PhI+ shown in Figure 2a exhibits an extremely fast decay, which can be well fitted by a single exponential decay convoluted with the cross-correlation function:

S PhI (t ) = C d exp( − t τ ) ⊗ g(t ) (1). +

7



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Where Cd corresponds to the coefficient of the decay component, τ is the decay time for PhI+, and g(t) is a Gaussian cross-correlation function with full width at half maximum (FWHM) of 200 fs. The cross-correlation function is used in the subsequent deconvolution of the time-resolved data. We have defined zero-of-time as the pump-probe delay time corresponding to the instantaneous rise of the experimental decay function used to fit the parent ion transient. A lifetime of 76±10 fs is extracted from the fitting.

Figure 2. Femtosecond pump-probe transient ion signals of (a) parent ions PhI+, (b) phenyl ions Ph+ with 200 nm pump pulse (0.4 µJ/pulse) and 298.2 nm probe pulse (1.8 µJ/pulse). The black dots represent the experimental data, the black lines represent their best obtained fittings, and the colored lines show the components of the fittings.

Unlike parent PhI+ transient, in addition to the decay part, Ph+ transient in Figure 2b displays a clear plateau at longer delay time, suggesting the existence of two different generation schemes for Ph+ fragments. Around zero-of-time, two probe photons are energetically required from the excited PhI to reach the appearance energy for Ph+ fragment (10.55 eV, from NIST29). The Ph+ transient collected from the dissociative ionization channel is expected to behave the similar decay with the PhI+ 8


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transient. However, as shown in Figure 2b, Ph+ transient displays a biexponential decay behavior dominated by an extremely fast decay component similar to PhI+ transient near zero-of-time. The plateau at longer delay time is due to the appearance of the phenyl radicals resulting from the neutral dissociation process. In order to extract the correlated time constants, two exponential decays and a rise function are required to obtain a good fit for Ph+ transient, S Ph (t ) = {C d 1 exp(− t τ 1 ) +

+ C d 2 exp(− t τ 2 )

(2).

+ C r[1 − exp(−t / τ Pr )]} ⊗ g(t )

Here, τ1 and τ2 are the decay times of the Ph+ fragment, τPr is the rise time of the phenyl radicals, Cd1, Cd2 and Cr are the coefficients of the corresponding functions. We extracted two decay time constants of τ1 = 75±10 fs, τ2 = 540±40 fs, and a rise time of τPr = 290±90 fs. Among these, τ1 of Ph+ transient agrees well with the decay time of PhI+ transient, but τ2 is absent in PhI+ transient, which we will discuss later for further details. The photodissociation of PhI produces phenyl radicals and iodine atoms of spin-orbit excited state I*(2P1/2) and ground state I(2P3/2). Under our experiment condition, the probe wavelength of 298.2 nm was chosen to directly measure the photodissociation lifetime and provided a direct view of the process of bond breakage. By recording the fragment I+ signals as a function of the time delay between the photolysis and the probe pulses, we measured the buildup time for the formation of the I(2P3/2) photoproduct. Figure 3 shows the I+ transients collected at different probe intensities, and the measured timeprofiles present strong dependence on the probe intensity. As shown in Figure 3a, the shape of the I+ transients changes with the increase of the probe intensity. When higher probe intensity is used, an extra component peaking near zero of the delay time is observed (shown as the blue dashed line in Figure 3a) due to absorption of more probe photons at higher intensity. This extra component, which comes from the fragmentation of the PhI+, has the same decay rate with the parent ion. Careful checking the timeprofiles taken at different probe intensities, we find that the shape of I+ transient is also affected by the 9



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saturation effect of the probe beam. The rise time of I+ transient becomes shorter as the probe pulse intensity increases, as shown in Figure 3a. Such saturation effect is problematic when determining the rise time of the fragments, since the measured rise time in the presence of saturation would be erroneously small. After carefully checking the I+ transients measured at various probe light intensities, we found that the measurement at 1.8 µJ/pulse intensity has a good signal to noise ratio (SNR) without saturation effect. Therefore, in order to obtain accurate rise time of I+ fragment, we performed the measurement of I+ transient at low pump (0.4 µJ/pulse) and probe (1.8 µJ/pulse) intensity to surely avoid the saturation effect, and extracted the rise time of τIr = 1.2±0.2 ps for the I(2P3/2) fragments, as shown in Figure 3b.

Figure 3. (a) Femtosecond pump-probe transient ion signals of fragment ions I+ taken at different probe intensities. Under our experimental condition, I(2P3/2) fragments are resonantly detected at 298.2 nm probe pulse through (2+1) REMPI process. (b) I+ transient measured with 1.8 µJ/pulse probe intensity in order to avoid saturation effect. The solid lines represent their best obtained fittings.

3.2. Theoretical calculations. Theoretical calculations on the photodissociation of PhI have been performed to interpret the observed data. The dissociation coordinates of both ground state and low-lying excited states were constructed in the S0 state. The 10


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CASPT2/cc-pVTZ optimized S0 geometry of PhI was compared to the previous MP2,6 CASSCF30 optimized results and available experimental values31 in Table 1. Both CASPT2 and MP2 optimized results agree well with the corresponding experimental values. Considering both static and dynamic correlation, the CASPT2 energy at the CASPT2 optimized geometry should be more reliable. The Ph ring was frozen at the CASPT2 equilibrium ground state geometry for all calculations along the C-I coordinate. The PECs along the C-I reaction coordinate for the ground state and first 18 excited states of PhI were calculated using SOF CASPT2 (12, 10)/cc-pVTZ method. The adiabatic output of the SOF CASPT2 ab initio calculations shows that multiple true and avoided crossings exist and the avoided crossings are in most cases very narrow. Therefore, for convenience of discussing the photodissociation mechanism, we constructed locally quasi-diabatic potential curves, as shown in Figure 4. According to the calculations, four strong repulsive curves (11B1, 13B1, 21B2, 23B2) and one quasi-bound curve (33A1) correlate to [Ph(X) + I(2P)] products at infinite separation, and six states (21A1, 21A2, 23A1, 23A2, 21B1, 23B1) correlate asymptotically to [Ph(A) + I(2P)] products, as shown in Figure 4. There are also ππ* excited states with minimum energies at RC-I ≈ 2 Å correlating with a higher excited state of Ph radical and I(2P) atoms. However, those are above the energy that 200 nm pump laser could reach, and we will ignore those exit channels.

Table 1. The main bond distances (in Å) of the S0 state of PhI.

Method MP2i) CASSCFii) CASPT2 Expiii)

C1-I 2.062 2.140 2.064 2.05

C1-C2 1.395 1.388 1.394 1.394

C2-C3 1.393 1.389 1.394 1.391

i)

Ref. 6 Ref. 30 iii) Ref. 31 ii)

11


C3-C4 1.393 1.389 1.393 1.393


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Figure 4. The calculated diabatic spin-orbit-free potential energy curves along RC-I for the ground and first 18 excited states of iodobenzene using SOF-CASPT2 (12,10)/cc-pVTZ calculations.

To study the effect of spin-orbit coupling, the absorption spectra of low lying SOC states composed of 11 singlet and 11 triplet SOF states were calculated by the SA-CASSCF approach. For the SOC states with vertical excitation energies (Tv) close to or below 200 nm, their Tv values, the components of the SOF states, the transition characters at the minimum (RC-I=2.064 Å), and oscillator strengths (f) are listed in Table 2. In the spin-orbit coupling picture, the benzene ππ* transitions and nπ* transitions are mixed with the alkyl like nσ* transitions. For states with significant mixing, percentages of mixing are also shown in Table 2 (only the SOF states with more than 5% contribution to the related SOC states are listed). It is worthwhile noting that the f values in some spin components of the triplet states increase to appreciable extent (such as 2B2, 5B1, 7B2 etc.), which means that the singlet-triplet excitation is enabled due to the spin-orbit coupling.

12


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Table 2. Calculated vertical excitation energies (RC-I = 2.064 Å), weights of the respective SOF states, associated electronic transitions and oscillation strengths at SOC-CASPT2(12/10)/cc-pVTZ level.

SOC states

vertical excitation Energy (Tv)/eV

1A1

0

electronic transitions

transition

99.4% 1 1A1

0

3

oscillation strength (f) 0

1A2

3.89

97.8% 1 A1

ππ*

0

1B1

3.89

99.3% 1 3A1

ππ*

4.0×10-4

1B2

3.89

99.0% 1 3A1

ππ*

3.0×10-4

2A1

4.04

78.0% 1 3B1; 21.8% 2 3B2

πσ*; nσ*

2.0×10-3

2A2

4.04

77.5% 1 3B1; 20.4% 2 3B2

πσ*; nσ*

0

2B2

4.12

93.9% 1 B1;

5.3% 2 B2

πσ*; nσ*

2.1×10-2

2B1

4.35

54.9% 2 3B2; 43.1% 11B1

nσ*; πσ*

0

3A1

4.44

99.8% 1 3B2

ππ*

0

3

ππ*

0

3

3A2

4.44

3

1

99.6% 1 B2

3B1

4.44

99.3% 1 B2

ππ*

3.0×10-4

4B1

4.45

98.4% 2 3A1

ππ*

2.0×10-4

3B2

4.45

99.8% 2 3A1

ππ*

0

3

4A2

4.45

99.6% 2 A1

ππ*

0

4B2

4.68

99.8% 1 1B2

ππ*

4.0×10-2

5A2

4.71

78.4% 2 3B2; 20.9% 13B1

4A1 5B1 5B2 5A1

4.74 4.86 5.32 5.65

3

3

77.2% 2 B2; 21.6% 1 B1 1

3

56.1% 1 B1; 42.5% 2 B2 1

92.8% 2 B2; 1

91.3% 2 A1;

3

5.3% 1 B1 3

8.5% 2 A2

1

nσ*; πσ*

0

nσ*; πσ*

0

πσ*; nσ*

0.10

nσ*; πσ*

0.46

ππ*; nπ*

7.26

6A2

6.23

99.9% 1 A2

πσ*

0

6A1

6.23

99.9% 1 3A2

πσ*

1.0×10-4

6B1

6.23

99.9% 1 3A2

πσ*

0

πσ*

1.0×10-4

nπ*

0

3

6B2

6.23

99.8% 1 A2

7A2

6.36

45.9% 2 1A2; 43.9% 2 3B1

7B2

6.36

51.5% 2 A2; 35.5% 2 B1; 11.9% 3 A1

nπ*; ππ*

1.0×10-2

7A1

6.37

89.2% 2 3B1;

nπ*

2.4×10-3

7B1

6.38

71.9% 2 3A2; 27.4% 33A1

8A2

6.41

3

3

3

9.8% 2 1A2

3

1

3

3

53.1% 2 B1; 26.0% 2 A2; 20.4%

nπ*; ππ*

2.1×10-3

3

nπ*; ππ*

0

3

3 A1

8B2

6.41

60.3% 2 B1; 20.5% 2 A2; 18.2%

3 A1

nπ*; ππ*

4.5×10-3

8A1

6.55

81.6% 2 3A2;

2 1A1

nπ*; ππ*

0.69

ππ*; nπ*

1.3×10-3

8B1

6.68

3

9.9% 2 3B1;

8.4%

3

72.5% 3 A1; 27.3% 2 A2

According to the absorption spectra that we calculated by the SA-CASSCF approach, 4B2, 5B1, 5B2, 5A1, 7B2 and 8A1 states, located at 4.68, 4.86, 5.32, 5.65, 13



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6.36 and 6.55 eV, respectively, are the main contributors of ultra-violet spectrum of PhI with larger oscillator strengths of 0.04, 0.1, 0.46, 7.26, 0.01, and 0.69, respectively. As presented in the previous work by Kimura and co-workers,1 the ultra-violet spectrum of PhI was decomposed into six single bands and designated from A to F bands centering at 4.83, 5.45, 6.20, 6.56, 7.12 and 7.55 eV, respectively. Considering the excitation energies and transition symmetries in Kimura’s research1, we can safely assign 4B2, 5A1, 7B2 and 8A1 states to the major compositions of A, B, C, D bands, respectively, and the pump wavelength of 200 nm that we used for our research work is able to excite PhI to 7B2 state of the C band.

Figure 5. Spin-orbit resolved PECs as functions of RC-I calculated at the SOC-CASPT2 (12/10)/cc-pVTZ level, based upon the CASPT2 energies depicted in Figure 4. The inset shows the PECs of states contributing to the dynamics involved in the photodissociation.

In order to refine the PECs with spin-orbit interactions, we have computed PECs of the SOC states as functions of RC-I at the SOC-CASPT2 (12/10)/cc-pVTZ level, which mostly agree well with those reported previously.6,30,32 Figure 5 (and the inset) shows the locally quasi-diabatic PECs constructed from the adiabatic output of the computations by careful checking the energies and wavefunction coefficients and the 14


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inset shows the PECs of states contributing to the dynamics involved in the photodissociation. The assumption of diabatic behavior is reasonable because the avoided crossings are narrow. In the dissociation limit defined at RC-I = 4.5 Å, the energy difference between the I(2P3/2) and I*(2P1/2) states is 0.94 eV, which agrees perfectly with the experimental value of 0.94 eV.33 Our dissociation energy (De) of S0 is estimated to be 3.2 eV from the average energies of the seven SOC states converging to the [Ph(X) + I(2P3/2)] limit which agrees with the experimentally detected 2.89 eV for PhI18 with an error of about 10%.

4. DISCUSSION Upon photo excitation at 200 nm, PhI molecules possess internal energy of 6.2 eV. Considering the typical error of CASPT2 computed excitation energies is less than 0.3 eV,34 6A1, 6B2, 7B2, 7A1, 7B1 and 8B2 states are possible states that 200 nm photon can reach according to their Tv and f values shown in Table 2. However, the f values of 6A1 and 6B2 states are much smaller compared with other states, and they cannot be important doorway optical states. Considering the Tv and f values listed in Table 2, the pump pulse of 200 nm mainly excites PhI to 7B2 state but the initial excitation of the 7A1, 8B2 and 7B1 is also taken into account. In the following, we discuss the photodissociation dynamics and rationalize the observed time constants. 4.1. PhI+ and Ph+ transient. As Figure 5 (and the inset) shows, 7B2 state is a quasi-bound state with a barrier existing on its PEC along the C-I dissociation coordinate. Upon 200 nm radiation, PhI molecules are mainly excited to 7B2 state, which crosses with 7A1 and 8B2 states at Frank-Condon (FC) region. The ultrafast decay observed for both PhI+ (τ = 76 fs) and Ph+ (τ1 = 75 fs) transient is assigned to the evolution along 7B2 state and then the subsequent IC taking place from 7B2 to 7A1 and 8B2 states. As Figure 5 (and the inset) shows, 7A1 and 8B2 are bound states with minima at RC-I ~ 2.7 Å and correlate to [Ph(A) + I(2P3/2)] products. When the excited PhI molecules relaxed from 7B2 state, the electronic energy is converted into high vibrational energy of 7A1 and 8B2 states, which is enough to excess the dissociation energy for [Ph(A) + I(2P3/2)] products. As we know, the bonds in the ions are generally 15



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weaker than those of the neutral molecules. Therefore, the decomposition of PhI+ is reasonably complete considering the vibrational energy remains in the PhI+ upon ionization from 7A1 and 8B2 states. The slower decay component (τ2 = 540 fs), which is absent in PhI+ transient and only observed for Ph+ transient, is thus safely assigned to the lifetimes of 7A1 and 8B2 states. 7A1 and 8B2 states themselves are also possible doorway optical states at 200 nm. With 200 nm pump, PhI molecules are initially excited to high vibrational excited states of 7A1 and 8B2 states, and then decay through IVR before they dissociate and generate [Ph(A) + I(2P3/2)] products. As mentioned above, due to complete decomposition of PhI+, 7A1 and 8B2 states exclusively contribute to the slower decay component (τ2 = 540 fs) of Ph+ transient. As another photo doorway state at 200 nm, 7B1 state resembles to 7B2 state at FC region, both having a barrier slightly higher than the pump photon. 7B1 state crosses with 7A1 and 8B2 states, just as 7B2 does. Therefore, the excited PhI of 7B1 state has similar relaxation channels to those of 7B2 state, which is undistinguishable from 7B2 state as for the contribution for PhI+ and Ph+ transients. 5B2 state is also a possible decay channel since it cross with 7B2 and 7B1 states at RC-I ~ 1.9 Å. However, as shown in Table 2, 7B2 and 7B1 are triplet states with transition characters mainly to be nπ* and ππ* while 5B2 is singlet state whose dominant component is 21B2 repulsive nσ* SOF state at FC region. After intersystem crossing with 7B2 and 7B1 states, the dissociation of PhI at 200 nm takes place via the repulsive 5B2 state and leads to [Ph(X) + I(2P3/2)] products. This is a typical Herzberg type I predissociation,35 and its dissociation rate is dependent on the couplings between the two crossing states. On account of the relatively slow decay rate for the transitions from the triplets states (7B2 and 7B1) to the singlet state (5B2), the excited PhI molecules on 7B2 and 7B1 states therefore prefer to decay through ICs to 7A1 and 8B2 states, which is much easier than to decay through ISCs to 5B2 state. As a result, we have not observed the contribution of the 5B2 state to the PhI+ transient in our experiment. Another thing we should mention is, 7A1 and 8B2 states also cross with 7B2, 8A2 16


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and 8B1 states at RC-I ~ 2.4 Å, whose PECs correlated with the [Ph(X) + I*(2P1/2)] production. Therefore, the crossings from the 7A1, 8B2 states to 7B2, 8A2 and 8B1 states leads to the production of I*(2P1/2), which makes 7A1, 8B2 states responsible for both the [Ph(A) + I(2P3/2)] and [Ph(X) + I*(2P1/2)] dissociation channels. But due to the bad SNR for the plateau part of Ph+ in Figure 2b, we cannot accurately extract two rise time constants for Ph(A) and Ph(X) radicals. Henceforth, only one rise time constant (τPr = 290 fs) with large error is obtained to denote the buildup time of the phenyl radicals from the neutral dissociation. This re-crossing from 7A1 and 8B2 to 7B2, 8A2 and 8B1 states also explains that the lifetime of 7A1, 8B2 states (τ2 = 540 fs) observed for Ph+ transient is relatively shorter than the buildup time (τIr = 1.2 ps) for I(2P3/2) production. 4.2. I+ transient. As Figure 3 shows, the buildup of the I+ transient is slow, reaching its plateau at about 4 ps, indicating an indirect dissociation channel. In order to discuss the formation of I(2P3/2) fragment, we only focus on the correlated states whose PECs lead to I(2P3/2) products.

Table 3. The selected ground and excited states of PhI are split into three groups according to their dissociation products.

SOC states

I/I*

1A1 2A1, 2A2 2B1, 2B2 5A2 5B1, 5B2

Ph(X) + I

4A1 7B2 8A2 8B1

Ph(X) + I*

7A1, 7A2 7B1 8B2

Ph(A) + I

According to the different exit channels that 200 nm pump laser could reach, 19 states are split into three groups, as shown in Table 3. The ground 1A1 and lower 17



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seven states (2A1, 2A2, 2B1, 2B2, 5A2, 5B1, 5B2) with their repulsive PECs diabatically converge to the [Ph(X) + I(2P3/2)] dissociation limit. The middle four states (4A1, 7B2, 8A2, 8B1) diabatically converge to the [Ph(X) + I*(2P1/2)] dissociation limit. And the upper eight states (7A1, 7A2, 7B1, 8B2, 5A1, 3B2, 3B1, 4A2) diabatically converge to the [Ph(A) + I(2P3/2)] dissociation limit, but four of them (5A1, 3B2, 3B1, 4A2) have barriers much higher than 6.2 eV and we will ignore these exit channels in this paper. However, as shown in Figure 5 (and the inset), among those seven lower states, only 5B2 state crosses with the doorway optical states of 7B2 and 7B1, but as discussed above, it can be excluded considering the absence of slow decay component in PhI+ transient. 7A1, 7A2, 8B2, 7B1 states are also candidates whose PECs lead to I(2P3/2) products accompanying with Ph radicals of excited state, as listed in Table 3, and we ignore the contribution of the 7A2 state considering its f value (f = 0). As for 7A1 and 8B2, the photodissociation originated from them have been discussed above. The 200 nm pump laser excites PhI to high vibrational states of 7A1 and 8B2 states, with vibrational energy in the phenyl ring modes. Then the C-I mode becomes excited though IVR and leads to the cleavage of the C-I bond. The rise time of 1.2 ps measured for I+ transient reflects the overall rate of the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(2P3/2) production. Due to the crossings with 7A1 and 8B2 states at FC region, 7B1 and 7B2 states are also responsible for the generation of I(2P3/2). Considering the ultrafast decay (τ1 = 75 fs) through IC from 7B1 and 7B2 states to 7A1 and 8B2 states is negligible compared with IVR of the benzene ring, we expect the formation time of the I(2P3/2) fragments originated from 7B1 and 7B2 states is comparable with that of 7A1 and 8B2 states.

5. CONCLUSION The photodissociation dynamics of PhI of highly excited states near 200 nm has been investigated using femtosecond time-resolved ion yields technique. With the aid of high-level, spin-orbit resolved ab initio calculations of one-dimensional potential energy curves, detailed photodissociation routes are discussed. Comparing our 18


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calculated absorption spectra with Kimura’s measurement, we identify 4B2, 5A1, 7B2 and 8A1 states with the major compositions of A, B, C, D bands, respectively. Pump beam of 200 nm enables us to explore the C band photodissociation dynamics of PhI. Upon 200 nm excitation, 7B2, 7A1, 7B1 and 8B2 states are possible doorway optical states. PhI molecules on 7B2 and 7B1 states decay to 7A1 and 8B2 states through IC in 75 fs, and the electronic energy is converted into high vibrational energy of 7A1 and 8B2 states, leading to the complete decomposition of PhI+. 7A1 and 8B2 states themselves are also doorway optical states with high vibrational energy in the phenyl ring modes, they decay through IVR before dissociation and generate [Ph + I/I*] products, and the lifetimes of 7A1 and 8B2 states are measured to be 540 fs. The C-I mode becomes excited through IVR, which leads to the cleavage of the C-I bond. The rise time of 1.2 ps measured for I+ transient reflects the overall time for the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(2P3/2) production.

AUTHOR INFORMATION Corresponding Authors *

Y. Tang. E-mail: [email protected].

*

B. Zhang. E-mail: [email protected].

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program) under Grant No. 2013CB922200, and National Natural Science Foundation of China under Grant Nos. 11574351, 21327804, 21503270.

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Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene

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