Doppler-free two-photon absorption spectroscopy of vibronic excited states of naphthalene assisted by an optical frequency comb UNIV. of Electro-Communications AFukuoka UNIV. BKyoto UNIV. Akiko NISHIYAMA Kazuki NAKASHIMAA Masatoshi MISONOA Masaaki BABAB

Introduction ～ High-Resolution Spectroscopy Singlet state Triplet state T1 T2 T3 ●Doppler-free spectroscopic technique → Rotationally resolved spectra 　→ Detailed structures of excited states S0 S1 S2 S3 ●Precise spectroscopic system 　→ Detection of small effect 　… Frequency shift, Line broadening, Zeeman effect, … 　→ Dynamics in excited states ~ 1015 Hz ●Requirement for the system accuracy and resolution ～ 10-9 = 1 MHz

Target ～ Excited States of Naphthalene <Two-Photon Transitions of Naphthalene> n4(b1u)+n7(ag) x z 2545 cm-1 Lemon n4(b1u)+n8(ag) 2260 cm-1 Yellow n4(b1u) 1560 cm-1 Orange S11B1u Wavelength [nm] 280 290 300 310 S01Ag

Motivation ～ Study of Line Broadening Effect due to IVR <Fluorescence spectrum> Excess energy > 2000 cm-1 Intramolecular Vibrational energy Redistribution (IVR) becomes dominant interaction Excess energy 1560 cm-1 Orange In this study, We observe beginning of the IVR phenomena precisely. 2260 cm-1 Yellow 2545 cm-1 Lemon S. M. Beck, et al., J. Chem. Phys. 74, 43 (1981).

Experimental Setup linewidth of 100 kHz scan speed of 2 MHz/sec free-running PD Naphthalene Cell CW dye Laser PBS Photomultiplier Tube I2 λ/4 AOFS APD RF Synthesizer Ti:S Comb Computer Photonic Crystal Fiber Diffraction Grating Frequency Counter stabilized to GPS disciplined clock AOFS: Acousto-Optic Frequency Shifter EOM: Electro-Optic Modulator APD: Avalanche Photodiode PBS: Polarizing Beam Splitter

Doppler-free two-photon absorption spectroscopy Mirror Mirror hf(1 + u/c) u 2hf f f(1 - u/c) f(1 + u/c) hf(1 - u/c) - Doppler free - Enhancement of light intensity for excitation - Selection rule is different from one photon absorption - Transition in UV region is excited by a visible laser - All molecules contribute to the signal independent of their velocities

Experimental Setup linewidth of 100 KHz scan speed of 2MHz/sec free-running PD Naphthalene Cell CW dye Laser PBS Photomultiplier Tube I2 λ/4 AOFS APD RF Synthesizer Ti:S Comb Computer Photonic Crystal Fiber Diffraction Grating Frequency Counter stabilized to GPS disciplined clock AOFS: Acousto-Optic Frequency Shifter EOM: Electro-Optic Modulator APD: Avalanche Photodiode PBS: Polarizing Beam Splitter

Optical Frequency Comb Cs atomic clock on GPS satellite fbeat < 10-11 Frequency fCEO frep fn flaser flaser = nfrep + fCEO + fbeat

Experimental Setup linewidth of 100 kHz scan speed of 2 MHz/sec free-running PD Naphthalene Cell CW dye Laser PBS Photomultiplier Tube I2 λ/4 AOFS APD RF Synthesizer Ti:S Comb Computer Photonic Crystal Fiber Diffraction Grating Frequency Counter stabilized to GPS disciplined clock AOFS: Acousto-Optic Frequency Shifter EOM: Electro-Optic Modulator APD: Avalanche Photodiode PBS: Polarizing Beam Splitter

Frequency Measurement of Scanning Laser fdye (a) CW dye frep (a) frep PBS time λ/4 fAO (b) (c) AOFS (b) frep Ti:S comb time (d) fdye + fAOM (c) frep 40.6 41.0 distribution fbeat fbeat [MHz] frep frep time fbeat (d) time Accuracy ～ 10s kHz A. Nishiyama, et al., Opt. Lett. 39, 4923 (2014).

Measurement of Two-Photon Spectrum of Naphthalene fAO [MHz] 1080 1000 41.2 fbeat [MHz] 40.6 16788.515 16788.53 1560 cm-1 Naphthalene two-photon absorption [arb. unit] Orange 33577.03 33577.06 Wavenumber [cm-1] A. Nishiyama, et al., J. Mol. Spectrosc. 318, 40 (2015).

Target ～ Excited States of Naphthalene <Two-Photon Transitions of Naphthalene> n4(b1u)+n7(ag) x z 2545 cm-1 Lemon n4(b1u)+n8(ag) 2260 cm-1 Yellow n4(b1u) 1560 cm-1 Orange S11B1u Wavelength [nm] 280 290 300 310 S01Ag

Observed Naphthalene Spectra Lemon origin n4(b1u)+n7(ag) Excess energy = 2545 cm-1 Yellow origin n4(b1u)+n8(ag) Excess energy = 2260 cm-1 Orange origin n4(b1u) Excess energy = 1560 cm-1 Observed rovibronic lines are assigned as Q(Ka)Q(J) transitions

Observed rovibronic spectra ～ Q(Ka)Q(J) transitions J=26, Ka=0,1, Kc=26 J=25, Ka=1,2, Kc=24 J=24, Ka=2,3, Kc=23 Orange n4(b1u) Yellow n4(b1u)+n8(ag) Lemon n4(b1u)+n7(ag)

Molecular constants Tentative value S1B1u [cm-1] Orange band n4(b1u) Yellow band n4(b1u)+n8(ag) Lemon band n4(b1u)+n7(ag) A 0.101 380 6 (3) 0.101 653 (16) 0.103 206 (48) B 0.040 434 0 (2) 0.040 431 3 (25) 0.040 125 0 (58) C 0.028 932 6 (2) 0.028 921 72 (16) 0.028 899 20 (52) Dj 1.35 (10) ×10-9 4.1 (10) ×10-7 -1.9 (8) ×10-7 Djk -1.26 (5) ×10-8 -2.0 (3) ×10-6 2.0 (9) ×10-7 Dk 1.52 (4) ×10-8 1.6 (2) ×10-6 -8.8 (12) ×10-8 dj 3.51 (8) ×10-9 2.1 (5) ×10-7 -9 (4) ×10-8 dk 1.03 (1) ×10-7 -7.2 (9) ×10-7 -4 (5) ×10-8 Origin 33578.03877 34279.2122 34562.6819 RMS 0.000028 0.00029 line 963 287 121 S0Ag [cm-1] Ground state n=0 O. Pirali, et al., Phys. Chem. Chem. Phys. 15, 10141 (2013). 0.104 051 836 (124) 0.041 127 33 (37) 0.029 483 552 (140) Dj×109 0.528 (49) Djk×108 0.1206 (145) Dk×108 0.5648 (112) dj×109 0.1752 dk×107 0.01951

Interaction between excited states in Orange band ＜Frequency shifts＞ Ka + Kc = J ＜Change of linewidths＞ 33577.915 .916 Signal intensity [arb. unit] Wavenumber [cm-1] 2.48 [MHz] Ka + Kc = J+1 [MHz] J 30 10 20 Ka = 0 Ka = 1 Ka = 2 Ka = 3 Ka = 4 200 -200 Ka = 5 Ka = 5 J 30 10 20 [MHz] 2 1 3 Linewidths have no J value dependence Coriolis interaction 16

Linewidth of the Ka = 0 lines Averaged linewidth (Ka=0 series) Natural width (calculated from the life time※) 2.71 MHz 0.91 MHz Orange　n4(b1u) 3.37 MHz 1.71 MHz Yellow n4(b1u)+n8(ag) Lemon n4(b1u)+n8(ag) 3.51 MHz 1.63 MHz ※U. Boesl et. al. Chem. Phys. Lett, 42, 16 (1976). ・Transit-time broadening = 0.64 MHz ・Pressure broadening = 0.53 MHz (20 Pa) ・Laser linewidth×2 ＝ 0.26 MHz ・Natural width 1.43 MHz <Contribution to the spectral linewidth> Variation of life time was observed in rovibrational linewidths

Summary To analyze the intramolecular vibrational energy redistribution (IVR) in the excited state of naphthalene, we observed three two-photon transitions with excess energies (Ex) about 2000 cm-1, S11B1u n4(b1u) + n7(ag) ← S01Ag Ex = 2545 cm-1 S11B1u n4(b1u) + n8(ag) ← S01Ag Ex = 2260 cm-1 S11B1u n4(b1u) ← S01Ag Ex = 1560 cm-1 We analyzed rotational dependences of the linewidth, and observed variation of the life times in rovibronic linewidth. Analysis of Ka dependences of the linewidth is in progress.