1. Fukuoka Univ. A. Nishiyama, A. Matsuba, M. Misono Doppler-Free Two-Photon Absorption Spectroscopy of Naphthalene Assisted by an Optical Frequency Comb 1

2. Introduction ～ High Resolution Spectroscopy High Resolution and High Precision S1S1 S0S0 S2S2 T1T1 T2T2 Vibrational structure Rotational structure → Basic properties → Chemical reaction processes Detailed Structures and Dynamics in excited states of molecules 2

3. Introduction ～ Conventional Spectroscopic Systems EOM confocal etalon Etalon marker … Uncertainty ～ a few MHz Frequency scanning cw Laser Frequency scanning cw Laser Frequency High resolution spectroscopy … Resolution ～ linewidth of cw laser ・ Doppler-free two-photon absorption spectroscopy ‧ Supersonic jet ～ Ideal high resolution spectroscopic system ～ Resolution and Uncertainty < natural width ( ～ 1 MHz) 3

4. Absolute frequency f n = nf rep + f CEO （ n : mode number ） f CEO (Carrier-Envelope Offset) f rep (Repetition rate ) fnfn → Precise optical frequency ruler Spectral Intensity Optical Frequency Introduction ～ Optical Frequency Comb GPS clock ～ 10 -11 Comb stabilized to GPS clock … Uncertainty = a few kHz 4

5. ● Development of High Resolution Spectroscopic System Frequency measurement system with an optical frequency comb … High Precision < natural width ( ～ 1 MHz) Doppler-free two-photon absorption spectroscopy … High Resolution < natural width ( ～ 1 MHz) ● Measurement of detailed structure of naphthalene spectra in whole molecular band →Dynamics in excited states Aim of Our Study 5

6. Doppler-free Two Photon Absorption Spectroscopy Ti:S Comb Power : 400 mW 600-1200 nm Ti:S Comb Power : 400 mW 600-1200 nm PC λ/4 λ/2 PBS PCF Single mode cw dye Laser Power : 1300 mW Linewidth: 100 kHz Single mode cw dye Laser Power : 1300 mW Linewidth: 100 kHz AOM Wavemeter RF Synthesizer PMT Photon Counting unit Frequency Counter (f rep, f CEO ) DG AOM: Acousto-optic modulator PBS : polarizing beam splitter PCF : photonic crystal fiber DG : diffractive grating APD: avalanche photo diode PMT: Photomultiplier tube APD Iodine Saturated Absorption Spectroscopy GPS clock Experimental Setup Frequency Counter (beat frequency) 6

7. 0 Spectral Intensity 160 Frequency[MHz] (RBW ： 300 kHz) f rep = 162.5 MHz Frequency Measurement with a Comb 01600 0 Spectral Intensity Frequency[MHz] f dye f rep fnfn f n+1 f dye - f n f n-1 - f dye Frequency f n+2 Laser spectra in optical region Beat spectrum in RF region 7

8. Doppler-free Two Photon Absorption Spectroscopy Ti:S Comb Power : 400 mW 600-1200 nm Ti:S Comb Power : 400 mW 600-1200 nm PC λ/4 λ/2 PBS PCF Single mode cw dye Laser Power : 1300 mW Linewidth: 120 kHz Single mode cw dye Laser Power : 1300 mW Linewidth: 120 kHz AOM Wavemeter RF Synthesizer PMT Photon Counting unit Frequency Counter (f rep, f CEO ) DG AOM: Acousto-optic modulator PBS : polarizing beam splitter PCF : photonic crystal fiber DG : diffractive grating APD: avalanche photo diode PMT: Photomultiplier tube APD Iodine Saturated Absorption Spectroscopy GPS clock Experimental Setup Frequency Counter (beat frequency) 8

9. Frequency Measurement with the Comb and an AOM f dye f dye +f AOM AOM shift f AOM f rep f rep /4 ～ 40 MHz 0160 [MHz] f rep fnfn f n+1 f n+2 f dye = f n - f AOM + f beat Absolute frequency of dye laser Uncertainty ～ 10 -11 ～ 5 kHz 9

10. 40.8 41.2 [MHz] AOM frequency shift Beat frequency 1.16 1.00 I 2 spectrum 17352.23.24.25.26 Wavenumber [cm -1 ] [GHz] [arb.unit] Saturated Absorption Spectrum of I 2 ～ P(62)17-1 Scan rate : 0.8 MHz/s, Gate time of the counter : 0.1 s Wave length : 576 nm Measurement time : 22 minutes a1a1 10

11. 520 206 808.470 520 206 790 520 206 825 [MHz] HWHM =1.5 MHz Absolute Frequency [MHz] Previous measurement [1] 520 206 808.45 (12) CIPM recommended value [2] 520 206 808.4 (2) [1] A. Nishiyama, et. al., J. Opt. Soc. Am. B 30, 2107 (2013). [2] T. J. Quinn, Metrologia 40, 103–133 (2003). Saturated Absorption Spectrum of I 2 ～ a 1 component 11

12. Doppler-free Two Photon Absorption Spectroscopy f dye u f dye (1-u/c)f dye (1+u/c) Dye Laser Linewidth:120 kHz Dye Laser Linewidth:120 kHz PMT PD λ/4 λ/2 20 Pa 58 W fluorescence 596 nm 298 nm S1S1 S0S0 － +－ + Hänsch-Couilland scheme 12

13. x z Two photon transition of naphthalene D 2h S11B1uS11B1u S01Ag S01Ag v 4 (C-C stretch)=1 : b 1u ～ 33578 cm -1 ～ 32018 cm -1 ～ 35804 cm -1 S21B3uS21B3u 1560 cm -1 Vibronic interaction →Allowed transition 13

14. Naphthalene Spectrum Wavenumber [cm -1 ] 33577.033577.533578.0 S 1 1 B 1u (v 4 =1 : b 1u ) ← S 0 1 A g (v=0) transition Band origin Scan rate : 2 MHz/s, Gate time of the counter : 0.2 s Photon count 1.16 1.00 [/0.2 s] Frequency shift [GHz] 14

15. Linewidth of Naphthalene spectra ＜ Q(K) Q(J) transitions （ ΔJ = 0, ΔK = 0 ） ＞ 5000 0 33577.9150.9160.9155 [cm -1 ] J = 14 Kc = J Ka = 0,1 HWHM = 1.24 MHz Natural width [1] = 0.91 MHz [1] U. Boesl et. al. Chem. Phys. Lett, 42, 16 (1976). S 1 1 B 1u (v 4 =1 : b 1u ) a b c [count] Resolution = 0.4 MHz 33577.85 [cm -1 ].90.95 1011121314151617J = 18 5000 Kc = J Ka = 0,1 0 Photon count [/0.2 s] 15

16. Linewidth of Naphthalene spectra ～ pressure broadening Scan rate : 1 MHz/s, Gate time of the counter : 0.1 s 10203040 0[Pa] 1.0 1.2 1.4 0.8 1.6 1.8 [MHz] Natural width [1] 0.91 MHz 33577.9150.9160.9155 [cm -1 ] [count] 16

17. Calculation of Rotational Constants S 1 B 1u (v 4 =1 ) This work [cm -1 ] S 0 A g (v=0 ) ※ [cm -1 ] A 0.101376580.104052 B 0.040436690.041127 C 0.028933180.029484 A-(B+C)/2 0.06669165(92) (B+C)/2 0.03468493(17) (B-C)/2 0.00575176(27) Dk×10 8 -7.41(77)1.87 Djk×10 8 2.95(30)0.118 Dj×10 9 0.95(48)0.580 Hk×10 10 -4.32(18) Hkj×10 10 3.34(13) Hjk×10 11 -6.63(35) Hj×10 12 1.70(31) dk×10 8 8.06(44)0.137 dj×10 9 3.03(31)0.159 hk×10 10 -4.08(26) hjk×10 13 -0.13(37) hj×10 14 0.27(17) ※ K. Yoshida, et al., J. Chem. Phys. 130, 194304 (2009). Least-squares fittings of the theoretical energies (840 lines) Ka + Kc = J + 1 [MHz] J 0301020 Ka = 0 Ka = 1 Ka = 2 Ka = 3 Ka = 4 Ka + Kc = J Obs.- Cal. 0 200 -200 0 200 -200 0 200 -200 0 200 -200 0 200 -200 17

18. ● We developed a precise frequency measurement system for frequency scanning cw lasers with an optical frequency comb. Precision = 5 kHz ● To realize high resolution narrower than the natural width, we adopted the Doppler-free two photon absorption technique. Resolution = laser linewidth ～ 100 kHz ● We applied the developed system to high resolution spectroscopy of naphthalene. ・ Theoretical analysis of obtained spectra in high rotational levels ・ Dynamics in excited states Summary 18

19. Range ： 17352.21 ~ 17352.26 cm -1 (1.5 GHz) Time ： 25 s f rep = 162 525 957.486 ± 0.005 Hz f CEO = 54.173 ± 0.004 MHz BPF1 BPF2 17352.22.23.24.25.26.27 Sat. Abs. [arb. unit] Wavenumber [cm -1 ] mode number n : 3200761 mode frequency f n : 520 206 800.38 [MHz] f rep f BPF1 f BPF2 System I ～ I 2 Saturated Absorption Spectrum ～ P(62)17-1

20. Ti:S Comb Power : 400 mW 600-1200 nm Ti:S Comb Power : 400 mW 600-1200 nm Dye Laser Power : 1300 mW Linewidth: 120 kHz Dye Laser Power : 1300 mW Linewidth: 120 kHz Wavemeter Frequency Counter (f rep, f CEO ) DG BPF : Band-Pass Filter ED : Envelope detector PCF : photonic crystal fiber APD I 2 Saturated Absorption Spectroscopy GPS clock PBS AOM APD Lock-in Amplifier Lock-in Amplifier PCF LPF ED BPF1BPF2 System I ～ Experimental Setup