The photoionization dynamics based on molecular pre-orientation

Accepted Manuscript The photoionization dynamics based on molecular pre-orientation

Meng-Lin Zhou, Jie Yu, Gao-Ren Wang, Shu-Lin Cong PII: DOI: Reference:

S1386-1425(19)30029-0 https://doi.org/10.1016/j.saa.2019.01.021 SAA 16707

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received date: Revised date: Accepted date:

30 September 2018 31 December 2018 14 January 2019

Please cite this article as: Meng-Lin Zhou, Jie Yu, Gao-Ren Wang, Shu-Lin Cong , The photoionization dynamics based on molecular pre-orientation. Saa (2019), https://doi.org/ 10.1016/j.saa.2019.01.021

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The photoionization dynamics based on molecular pre-orientation

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Meng-Lin Zhou, Jie Yu ∗ , Gao-Ren Wang, Shu-Lin Cong

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School of Physics, Dalian University of Technology, Dalian 116024, China

Abstract

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We study theoretically the photoionization dynamics of pre-oriented NaK molecule. Firstly, a THz laser pulse is utilized to orient the ground state molecule. And then the pump and probe laser pulses are used to excite and ionize the molecule, re-

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spectively. We study the influence of molecular orientation duration and degree on

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the ionization probability, angle-resolved photoelectron spectrum and photoelectron angular distribution(PAD). It is shown that we could obtain more stable ionization signal and PAD when the molecules are ionized in molecular orientation duration.

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We could increase the ionization probability and obtain more concentrated ionization signal and photoelectron distribution by increasing the orientation degree in the ground state. Moreover, we discuss the splitting pattern in the angle-resolved

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photoelectron spectrum.

Keywords: pre-orientation, pump-probe photoionization, ionization probability, angle-resolved photoelectron spectrum, PAD.

∗ Corresponding author. Fax:+86-0411-84709304. [email protected] Preprint submitted to Elsevier

15 January 2019

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Introduction

With the development of laser technology, the photoionization and PAD have aroused considerable interest of researchers over the past few decades. The femtosecond linearly polarized pulses and attosecond circularly polarized pulses have been employed to control the photoionization process [1–3].

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Kastner et al. studied experimentally the intermediate state dependence of photoelectron circular dichroism (PECD) in resonance-enhanced multi-photon ionization (REMPI) [4]. Giri et al. theoretically explored the interplay of fluc-

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tuations on single-photon ionization [5].

On the other hand, many researchers took more interest in controlling molec-

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ular orientation or alignment because of its widespread application in physics and chemistry [6–13]. Alignment implies that the molecular symmetry axis is parallel to a spaced-fixed axis (for example the polarization direction of the

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laser field), and orientation additionally requires the aligned molecules pointing to a particular direction [14]. In many applications, both higher orientation

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degree and longer orientation duration are simultaneous required. There are

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many schemes focusing on increasing the molecular orientation degree. Up to now, several laser fields have been used to obtain high orientation degree, such as THz laser field [15,16], two-color laser field [17–19], combination field

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[20,21] and so on. Dames et al. [22] and Yu et al. [23] presented a scheme to achieve an efficient long-lived field-free molecular orientation in 2005 and

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2012, respectively. The long duration can be used to probe electron dynamics via photoemission or high harmonic generation and vibrational dynamics. In recent years, researchers begin to study the combination of molecular orientation and ionization dynamics. Arasaki et al. presented a formulation of energy- and angle-resolved photoelectron spectrum for femtosecond pumpprobe ionization [24]. Zhang et al. studied the influence of molecular preorientation degree on the REMPI dynamics of LiH molecule [14]. In the present work, we investigate the ionization dynamics of NaK molecule 2

ACCEPTED MANUSCRIPT with pre-orientation. Firstly, we use THz laser pulse oriented the NaK molecule in the ground state, then femtosecond pump and probe pulses are adopted to excite and ionize the molecule. Ionization with different molecular orientation degree are calculated. We mainly discussed the influence of molecular preorientation on ionization probability, angle-resolved photoelectron spectrum and PAD.

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This paper is organized as follows. In Section 2, we give the theoretical methods of molecular orientation and photoionization. In Section 3, we show the calculation results and discuss the influence of molecular orientation duration and degree on photoionization dynamics. Finally, we give a summary

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Theoretical Approach

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2

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in Section 4.

P+

state (black solid line), excited A1 2 P+

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X1

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In our control scheme, three electronic states are considered: the ground

and the ionic ground X

P+

state (red dotted line) of NaK

state (green dashed line) of NaK + [25,26], as

shown in Fig.1(a). For convenience, X 1

P+

, A1

P+

and X 2

P+

are abbreviated

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to |Xi, |Ai and |Ii, respectively. A THz single-cycle pulse (THz SCP) is used to orient molecule in the |Xi state. A pump laser pulse is then employed to

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excite the oriented NaK molecule from the |Xi state to the |Ai state. After a delay time, a probe laser pulse is used to ionize the molecule. The polarization direction of the THz SCP, the pump and probe pulses are assumed to be paralleled to each other and are set to be the z -axis. In Fig.1(b), p~e is the momentum vector of the emitted photoelectron. θ=α, are the angle between the Na-K direction and the polarization direction of the laser pulses, and β represents the angle between the momentum of photoelectron p~e and the polarization direction of the probe laser pulse. 3

ACCEPTED MANUSCRIPT z (ε k ) k = T H z , p u m p , p r o b e

(b) E n e rg y [a .u .]

(a )

p e

β θ (α )

K

y N a x

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R[a.u]

P Fig. 1. (a) The potential energy curves of the ground electronic X 1 + state (black P P solid line), the excited A1 + state (red dotted line), the ionic ground X 2 + state

(green dashed line) and the process of orientation, excitation and ionization. (b)

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The polarization directions of the laser pulse, where θ and α are the angles between the Na-K direction and the polarization directions of THz SCP and pump/probe

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pulse, respectively, and β is the angle between the momentum vector of emitted photoelectron and the probe laser polarization direction.

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A. Molecular orientation

THz SCP is a typical laser pulse to control the molecular orientation due to its asymmetry. The molecular orientation dynamics is described by the

i~

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time-dependent Schr¨odinger equation ∂ ˆ χX (t) = H(t)χ X (t), ∂t

(1)

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where χX is the nuclear wave function of the |Xi state. The molecular Hamiltonian is given by [27–29]

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ˆ X (t) ˆ = TˆR + Tˆθ + VX (R) + W H(t) ~2 ∂ 2 1 ∂ ∂ ~2 ˆ X (t), =− (sin θ ) + VX (R) + W − 2 2 ∂θ 2μ ∂R 2μR sin θ ∂θ

(2)

where μ is the reduced mass, R the internuclear separation, and VX (R) the potential energy of the |Xi state. The field-molecule interaction can written as ˆ X (t) = −μXX (R)εTHz (t) cos θ, W

(3) 4

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D i p o l e m o m e n t [D ]

μ μ μ

X X X A A A

D D D

R[a.u.]

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Fig. 2. The transition and permanent dipole moments of NaK as a function of internuclear distance R. μXX (R) (black solid line) and μAA (R) (red dotted line) are permanent dipole moments in the ground and excited states respectively, μAX (R)

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(green dashed line) are transition dipole moments.

where μXX (R) are the molecular permanent dipole moments in the |Xi state,

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4 ln 2(t − tTHz )2 exp − cos[ωTHz (t − tTHz )], 2 TTHz

(4)

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εTHz (t) = ε0THz

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and the electric field of the THz SCP can be expressed as

where ε0THz , tTHz , TTHz and ωTHz are the peak intensity, central time, full width at half maximum (FWHM) and central frequency of the THz SCP, respectively.

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The degree of orientation hcos θi can be written as hcos θi(t) = hχX (R, θ, t)| cos θ|χX (R, θ, t)i.

(5)

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The time internal that |hcos θi| remains greater than or equal to 0.5 within a period is the orientation duration Δt [22,23]. The molecule always maintains

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a good orientation effect during this period. B. Multi-photon ionization To describe the ionization process of the oriented molecule driven by strong laser pulse, the ionization continuum state needs to be treated by using the discretization method [3,30]. The total Hamiltonian can be written as ˆ = H(t)

X b

ˆ b hΦb | + |Φb iH

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ˆ (t), dEn |Φn i(HI0 + En )hΦn | + W 5

(6)

ACCEPTED MANUSCRIPT where |Φb i (b = X, A) denotes the two bound electronic states of NaK, and |Φn i the orthonormal scattering states of the photoelectron with kinetic energy

ˆ (t) denotes the laser-molecule interaction En . W Wˆ(t) = −μij (t) cos α, i, j = X, A, I

(7)

where μij are the transition (i 6= j ) and permanent (i = j ) dipole moments.

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ˆ b and H ˆ 0 represent the Hamiltonians in the two bound electronic states The H I

of the NaK molecule and the ground state of NaK + , respectively. The total

X b

χb (R, α, t)|Φb i +

N −1 X n=0

χIn (R, α, En , t)|Φn i,

(8)

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|Ψ(R, α, t)i =

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molecular wave function can be expressed as

where χb (R, α, t) and χIn (R, α, En , t) are the nuclear wave functions in different electronic states. It is noted that the ionization continuum state χIn (R, α, En , t)

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is discretized into N discrete states with a small enough energy interval [31,32]. The minimum and maximum energies of the photoelectron allowed in the sim-

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ulation are E0 and EN −1 , respectively. The time evolutions of the nuclear wave

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functions χX , χA and χIn in different electronic states are obtained by numerically solving the time-dependent Schr¨odinger equation [33]. The electric field is given by

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#

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(t − tk )2 cos[ωk (t − tk )], k = pump, probe, εk (t) = ε0k exp −4 ln 2 Tk2

(9)

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where ε0k , tk , Tk and ωk are the peak intensity, central time, FWHM and central frequency of the pump and probe pulses in order. The delay time between the pump and probe pulses is τp = tprobe − tpump , and the delay time between the THz SCP and the pump pulses is τd = tpump − tTHz . The time-dependent populations of the |ii state is calculated by Pi (t) =

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dα sin α

Z

dR|χi (R, α, t)|2 , i = X, A.

6

(10)

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PI (En , α, t) = lim

t→∞

Z

dR|χIn (R, α, En , t)|2 .

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To compute numerically the PAD, the nuclear wave function χIn is expanded

χIn (R, α, En , t) =

X

χIn ,J (R, En , t)PJ (cos α),

J

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in the Legendre function PJ (cos α) as [34] (12)

where J is the rotational quantum number. The PAD is written as [24]

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dR|χIn ,J (R, En , t)PJ (cos β)|2 .

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dEn

Results and Discussions

(13)

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t→∞

Z

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Ω(β, t) = lim

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In our numerical calculation, the permanent dipole moments μXX (R) (black

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solid line), μAA (R) (red dotted line) and transition dipole moments μAX (R) (green dashed line) of the NaK molecule are obtained from Ref.[25], as shown in Fig.2. The initial state is taken to be the ground rovibrational |v = 0, J = 0i

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state of the ground electronic state at temperature T = 0 K. A. Molecular orientation by THz laser pulse The paraments of THz SCP used in our scheme are ε0THz = 9.0 × 107 V/m,

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tTHz = 0.0 ps, TTHz = 0.45 ps and ωTHz = 36 cm−1 . The integral area of the THz SCP within a period, as shown in Fig. 3(a), is zero for the chosen ωTHz and TTHz [35]. From Fig. 3(b), we can see that the molecule is oriented when

the THz SCP is over, and the orientation degree hcos θi oscillates at a period

Tosc ≈ Trot , where Trot = (2cBe )−1 = 175.56 ps is the rotational period of

the NaK molecule, the rotational constant Be = 0.095 cm−1 . The maximum orientation degree hcos θi = 0.647 in the positive direction and hcos θi = -0.625 7

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Electric Field [10-5 a.u.]

(a)

T im e[p s ]

T im e[p s ]

(d)

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Electric Field [a.u.]

(c)

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T im e[p s ]

Fig. 3. (a) The time evolution of a THz SCP with ε0THz = 9.0 × 107 V/m, tTHz =

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0.0 ps, TTHz = 0.45 ps and ωTHz = 36 cm−1 . (b) The time-dependent NaK molecular orientation induced by the THz SCP. (c) The molecular orientation versus time in

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a rotational period from -1.0 to 180.0 ps, where the seven reference delay times (τd1 − τd7 ) in detail discussion are marked as well. (d) The time evolutions of the

pump and probe pulses with ε0pump = 9.0 × 109 V/m, ωpump = 13010 cm−1 , tpump =

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0.01 ps (k = pump, probe).

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2.68 ps, ε0probe = 3.0 × 1010 V/m, ωprobe = 43000 cm−1 , tprobe = 2.69 ps and Tk =

in the negative direction. The effective durations of both positive and nega-

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tive orientation are Δt = 26.90 ps. The duration is long enough to control the subsequent excitation process.

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The molecular orientation in the range from -1.0 ps to 180.0 ps are shown in Fig. 3(c). In the following, we discuss the dependence of ionization on the molecular orientation. In order to compare the effects of orientation duration and different orientation degree on ionization dynamics, we mark seven delay times τdi (i = 1 to 7, named as Cases 1-7) in Fig. 3(c). B. The effect of orientation duration and degree on photoionization process In the calculation, we take the smallest energy E0 and largest energy EN −1 to 8

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(b)

(e )

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(a)

(c)

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(f)

Fig. 4. The ionization probability(a)(d), angle-resolved photoelectron spec-

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trum(b)(e) and PAD(c)(f) in Cases 4 and 5, (a)-(c) correspond to Case 4 and (d)-(f) to Case 5. The ionization processes take place out the orientation duration. The laser

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pulses used in calculation are shown in Fig. 3(d).

be 0.0 and 20.0 eV, respectively. N is optimally taken to be 100. Since the accurate value of μAI (R) is not found in literatures, it is taken as μAI = μAX /20 [36]. The results are relative intensity of photoionization signal, not the absolute probability of ionization. The electric fields of the pump and probe pulses used in the calculation are shown in Fig. 3(d). The laser parameters are taken to be the optimized values: ε0pump = 9.0 × 109 V/m, ωpump = 13010 cm−1 , ε0probe

= 3.0 × 1010 V/m, ωprobe = 43000 cm−1 , and Tk = 0.01 ps (k = pump, probe). 9

(a)

(d)

(b)

(e )

(c)

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(f)

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Fig. 5. The ionization probability(a)(d), angle-resolved photoelectron spec-

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trum(b)(e) and PAD(c)(f) in Cases 1 and 3, (a)-(c) correspond to Case 1 and (d)-(f) to Case 3. The ionization processes take place in the orientation duration. The laser

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pulses used in calculation are shown in Fig. 3(d).

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The delay time between the pump and probe pulses τp = 10 fs. Figures 4 shows the ionization probability, angle-resolved photoelectron spectrum and PAD for Case-4 and -5, in which ionization process takes place

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out the orientation duration. The ionization probabilities in Fig. 4(a) and (d) are 0.748 and 0.802, respectively. The peaks of ionization signal in Fig. 4(b) and (e) are situated near both α = 0◦ and α = 180◦ . The photoelectrons are

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mainly distributed near β = 0◦ and β = 180◦ in Fig. 4(c) and (f). In additional to this, PADs exhibit multiple angular nodes because of the low molecular orientation degree in the ground state (hcos θi = -0.241 for Case 4 and hcos θi = -0.403 for Case 5). Figures 5 shows the ionization probability, angle-resolved photoelectron spectrum and PAD for Case-1 and -3, in which ionization process takes place in the orientation duration. The ionization probabilities in Fig. 5(a) and (d) are 10

(b)

(c)

(d)

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Fig. 6. (a-d) The time-dependent populations of the ionization continuum state

calculation are shown in Fig. 3(d).

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Pion (t) in Cases 1(a), 2(b), 6(c) and 7(d) when τp = 10 fs. The laser pulses used in

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0.844 and 0.832, respectively. The ionization signals in Fig. 5(b) and (e) are

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mainly distributed in the vicinity of α = 0◦ . The photoelectrons in Fig. 5(c) and (f) are mostly distributed in a region near β = 0◦ , which means the photoelectron trends to move along the same polarization direction of the laser

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pulse.

Comparing Figures 4 and 5, we can find that ionization signals and pho-

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toelectrons are distributed in different directions when the ionization process occurs out the orientation duration. However, they are distributed in same direction when the ionization process occurs in the orientation duration. The ionization probabilities in Fig.5 are higher than those in Fig. 4. That means excited and ionized molecule during orientation duration could increase ionization probability and obtain more stable ionization signal and photoelectron distribution. To further illustrate the influence of orientation degree on ionization dynam11

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(c)

(d)

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(a)

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Fig. 7. (a-d) The angle-resolved photoelectron spectra PI (En , α) in Cases 1(a), 2(b), 6(c) and 7(d) when τp = 10 fs. The laser pulses used in calculation are shown in

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Fig. 3(d).

ics, four moments are selected. When τd1 =15.50 ps (Case 1), τd2 =26.94 ps (Case 2), τd6 =132.00 ps (Case 6), and τd7 = 146.00 ps (Case 7), as shown in

respectively.

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Fig.3(c), the orientation degrees hcos θi are 0.521, 0.647, -0.577 and -0.625,

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Figures 6 (a)-(d) show the time evolutions of populations in the ionization continuum state Pion (t) in Cases 1, 2, 6 and 7 in order. The total ionization probabilities are Pion = 0.844, 0.905, 0.877 and 0.892, respectively. We can see that higher orientation degree result in higher ionization probability, the conclusion is consistent with Zhang’s work [14]. The difference is the maximum ionization probability is less than 0.4 in Zhang’s work. The reason for the high ionization probability in this work is due to the large dipole moments of the NaK. Analyzing from formula (9), the scale of Pion is associated with 12

(b)

(c)

(d)

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(a)

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[ps]

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[ps]

Fig. 8. (a-d) The evolution of the excited state coherent ro-vibrational excited wavepacket with time for Cases 1, 2, 6 and 7. The laser pulses used in calcula-

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tion are shown in Fig. 3(d).

χi (R, En , α, t), and it is determined by μAI and ε0probe . The dipole moments of NaK are about ten times larger than LiH, thereby greatly increasing the

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ionization probability. In the case where the ionization pulse intensity is same, a system with larger dipole moments can obtain higher ionization probability.

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Figures 7 (a)-(d) show the angle-resolved photoelectron spectra just after the action of the probe pulse (t − tprobe = 10 fs) in Cases 1, 2, 6 and 7. In Figs. 7 (a) and (b), the ionization signals are mainly distributed in the angular region near α = 0◦ , corresponding to the positive orientation in the ground state. The signals in Fig. 7 (c) and (d) are mainly distributed in the angular region near α = 180◦ , corresponding to the negative orientation in the ground state. In short, we can steer the angular distribution of ionization signal by controlling the molecular orientation duration and degree. 13

(b)

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(d)

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Fig. 9. (a-d) The photoelectron angular distributions Ω(β) in Cases 1(a), 2(b), 6(c) and 7(d) when τp = 10 fs. The laser pulses used in calculation are shown in Fig.3(d).

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The time-delay dependent photoelectron spectra illustrate the molecular

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coherent excitation. Figures 8 (a)-(d) show the evolution of the excited state coherent ro-vibrational excited wavepacket with time for Cases 1, 2, 6 and 7. In Case-1 and -2, the wavepacket are mainly distributed in a region α = 0◦ -

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90◦ while the wavepacket are mainly distributed in a region α = 90◦ - 180◦ in Case-6 and -7. The wavepacket oscillates when pump laser acts. Figures 9 (a)-(d) show the PADs in Cases 1, 2, 6 and 7. The photoelectrons

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in Figs. 9 (a) and (b) are mostly distributed in a region near β = 0◦ , which means the photoelectron trends to move along the positive laser polarization direction. The photoelectron in Figs. 9 (c) and (d) are mostly distributed in a region near β = 180◦ , which means the photoelectron trends to move along the negative laser polarization direction. The distributions are more concentrated in Case-2 and -7. Case 2 corresponding hcos θi =0.647, Case 7 corresponding hcos θi =-0.625. Both of them are approach the maximum orientation degree. 14

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Fig. 10. Black solid line: the molecular orientation versus time in a rotational pe-

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riod from -1.0 to 180.0 ps. Red solid circle line: The parameter ξ charactering the

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asymmetry of PAD at selected delay time.

Fig. 11. (a) The photoelectron spectrum when τp =10 fs in Case 7. (b) The time evolutions of the populations of the |Xi state (black solid line) and |Ai state (red dashed line). The delay time between pump and probe pulses is τp =10 fs. The inset shows the details from 146.000 ps to 146.003 ps.

From the above analysis, in the orientation duration, with the increasing of orientation degree, the PAD are more concentrated. To characterize the asymmetry of the PAD, we define an parameter ξ as 15

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(c)

[eV]

(b)

146.2

146.3

146.4

Energy[eV]

Fig. 12. (a) The angle-resolved photoelectron spectra PI (En , α) in Cases 7. (b) The

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photoelectron spectrum when τp =30 fs in Case 7. The inset shows the details from 8.0 eV to 12.0 eV. (c) The time evolutions of the populations of the |Xi state (black solid line) and |Ai state (red dotted line). The delay time between pump and probe

   Ω(180◦ )   , Ω (0◦ ) − ◦ Ω(0 )

> Ω (180◦ ) < Ω (180◦ )

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ξ=

    Ω(0◦ )  ◦   + Ω(180◦ ) , Ω (0 )

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pulses is τp =30 fs.

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When the photoelectron mainly distributed around 0 ◦ , ξ =

(14)

Ω(0◦ ) Ω(180◦ )

is pos-

itive and large. When the photoelectron mainly distributed around 180 ◦ , ◦

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) is negative and |ξ| is large. The asymmetry parameter ξ is shown ξ = − Ω(180 Ω(0◦ )

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in Fig. 10 as red solid circle line at several selected moments in an entire orientation period. The molecular orientation is also shown in Fig. 10 by black solid line. It is found that the variation of the asymmetry parameter ξ reflects

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the variation of the molecular pre-orientation degree. Significant asymmetry of PAD appears when the pre-orientation degree is large. This indicates clearly

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the effect of orientation on PAD. As shown in Figs.7 (a)-(d), there are many peaks in the angle-resolved photoelectron spectrum. Taking Case-7 as an example, Fig. 11(a) displays the photoelectron spectrum, there are other small peaks near the main peak 3.838 eV. The location of these peaks corresponding to the splitting in Fig. 7(d), we think this is Autler-Townes Splitting [37], the reason for peaks near 0eV is generated by the superposition of pump and probe pulses, another peak near the 10eV corresponding two-photon ionization process. Fig. 11(b) displays the 16

ACCEPTED MANUSCRIPT time evolutions of the populations of the |Xi and |Ai states. For a clear analysis, the inset is the details from 146.000 to 146.003 ps, we can observe obvious Rabi oscillation which cause from the superposition of pump and probe laser pulses. In the following, we will discuss the influence of delay time τp on the angleresolved photoelectron spectrum. Fig. 12(a) shows the angle-resolved photo-

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electron spectra when τp =30 fs in Case 7, the separated peaks have disappeared. Fig. 12(b) and (c) show the photoelectron spectrum and the time evolutions of the populations of the |Xi and |Ai states in Case-7 when τp =30 fs. The photoelectron spectrum only exhibits two peaks in Fig. 12(b).

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The Rabi oscillations also disappear in Fig. 12(c). When τp =30 fs, pump and probe laser pulses no longer have overlapping parts. Laser pulse intensity is

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not strong enough and the Rabi oscillation is insufficient. In short, the split-

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ting affected by the delay time between pump and probe pulses.

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Conclusion

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Taking NaK molecule as an example, we have theoretically researched the

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influence of molecular orientation duration and degree on the photoionization dynamics. We firstly use a THz laser pulse to orient the molecule in the ground

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state. A pump laser pulse is then employed to excite the molecule. Finally the probe laser pulse is used to ionize the molecule. The results show that the ionization probability, angle-resolved photoelectron spectrum and PAD are closely related to the molecular duration and degree. For NaK molecule, the effective durations of both positive and negative orientation are 26.90 ps, which is long enough to allow one to control the subsequent ionization process and obtain the concentrated signal. When the ionization process takes place in the orientation duration, the ionization probabilities have some differences, distri17

ACCEPTED MANUSCRIPT bution pattern of the angular-resolved photoelectron spectra are similar and the photoelectrons are mostly distributed in a region near β = 0◦ (β = 180◦ ) for the positive (negative) molecular orientation. Besides we discuss the relationship between the asymmetry of PAD and molecular orientation. Moreover, as the degree of orientation increases, the ionization probability increases, and the photoelectron angular distribution becomes more concentrated. Finally,

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we discuss the splitting pattern in angle-resolved photoelectron spectrum induced by Rabi oscillation. This phenomenon can be observed when pump and

Acknowledgments

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5

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probe pulses are superimposed.

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This work is supported by the National Natural Science Foundation of China under Grant No. 11274056, 11374045 and 21473018, and the National Key

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R&D Program of China(No. 2018YFA0306503).

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[1] K. J. Yuan, A. D. Bandrauk, Phys. Rev. A 85 (2012) 053419.

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Graphical abstract

ACCEPTED MANUSCRIPT Highlight:

1. The long orientation duration can be used to maintain ionization signal stability. 2. The high orientation degree can be used to increase ionization probability. 3. AT Splitting in spectrum can be observed when pulses are superimposed.

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4. The asymmetry of PAD is related to molecular orientation.

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