2 Univ. of Electro-Communications High Resolution Spectroscopy of A 1B1u <- X 1Ag 810410 Band of Naphthalene Referenced to an Optical Frequency Comb Kazuki NAKASHIMA1 Akiko NISHIYAMA2 Masatoshi MISONO1 Thank you for chairman. Good morning everyone. I am Masatoshi MISONO from Fukuoka University, Japan. Today, I talk about our study on "High resolution spectroscopy of this band of naphthalene referenced to an optical frequency comb." 1 Fukuoka Univ. 2 Univ. of Electro-Communications

Introduction ●Dynamics of electronic excited states in polyatomic molecule - Basic properties of molecules - Chemical reaction processes ●Interactions between energy levels appears as minute effects - Frequency shift - Line broadening - Intensity anomaly - Zeeman splitting Optical Frequency The dynamics of electronic excited states in polyatomic molecules are one of the most significant topics in molecular science. The interactions between energy levels in the electronic excited states, appears as minute effects such as frequency shifts, line broadening, intensity anomalies, and Zeeman effects. To observe such minute effects, we need, high resolution spectroscopy, precise measure of optical frequency. ●Requirements High resolution spectroscopy Precise measure of optical frequency

Broadening of rovibronic lines due to IVR (Naphthalene) This time we study broadening of rovibronic lines of naphthalene due to Intra-molecular Vibrational energy Redistribution, IVR. These are fluorescence spectra from S1 states of naphthalene, observed 35 years ago. These small numbers are the excess energies of the excitations. As you see, for small excess energies, the lines are well resolved. But, from two thousand one hundred twenty two wavenumbers, the lines start to become broad, and, for two thousand five hundred seventy wavenumbers, the lines are broad and each line is not resolved. This time, we study on the onset of the IVR. We measure high resolution spectra of the wavenumbers around here. S. M. Beck, et al., J. Chem. Phys. 74, 43 (1981).

Two-photon transition of naphthalene n4(b1u)+n7(ag) 34563 cm-1 2545 cm-1 n4(b1u)+n8(ag) 34278 cm-1 2260 cm-1 n4(b1u) 33578 cm-1 1560 cm-1 S11B1u 32018 cm-1 x z Naphthalene has these intense two photon transitions to the S1 state. Up to now, we observed this transition, and analyzed, and we assigned rovibronic transitions. And, we found no IVR effect. These three transitions correspond to ... D2h S01Ag

Line broadening due to IVR Excess energy 1560 cm-1 ... these three positions. As you see, for this transition, the IVR does not occur. These two transitions just correspond to the onset of IVR. 2260 cm-1 2545 cm-1

Two-photon transition of naphthalene n4(b1u)+n7(ag) 34563 cm-1 2545 cm-1 n4(b1u)+n8(ag) 34278 cm-1 2260 cm-1 n4(b1u) 33578 cm-1 1560 cm-1 S11B1u 32018 cm-1 x z So, this time, we observe this transition. The excess energy is two thousand two hundred sixty wavenumbers. D2h S01Ag

Optical frequency comb flaser Cs atomic clock on GPS satellite fbeat Intensity Frequency fCEO frep fn Now, I explain the precise frequency measure. These years, optical frequency combs commonly used in the field of precise frequency measurement. We also use an optical frequency comb. Here, I explain the principle of optical frequency measurement briefly. An optical frequency comb is a pulse laser, and the repetition rate is frep. In the frequency region, the repetition rate corresponds to the separation of the modes. And the offset frequency, fceo arises due to the difference between the phase of carrier and that of pulse envelope. Then, the frequency of the n-th mode is expressed as n times frep plus fceo. When we measure the frequency of the beat note between the comb and another laser, we can obtain the frequency according to this equation. We stabilize the frequency of the comb by GPS clock. flaser = nfrep + fCEO + fbeat

Experimental setup (circular polarization) APD Naphthalene Cell EOM CW Dye Laser PBS PBS λ/4 583 nm 1.5 W I2 λ/4 PMT AOM APD RF Synthesizer Ti:S Comb Computer DG PCF Frequency Counter 600 - 1200 nm 400 mW This is the experimental setup. The light source for spectroscopy of naphthalene was single mode CW dye laser. The dye was rhodamine 6G, and the power was about one point five Watts. We adopted Doppler-Free Two-Photon absorption spectroscopy with Fabry-Perot resonator. The resonance frequency was locked to the laser frequency during the laser scan by Pound-Drever-Hall method. The fluorescence intensity was observed. The lower half is the frequency measurement system. We used Titanium Sapphire comb. A part of the dye laser beam was used for frequency measurement. Before overlapping, the dye laser frequency was shifted by an acousto-optic frequency shifter. And the beat frequency was measured. APD: Avalanche Photodiode PBS: Polarizing Beam Splitter AOM: Acousto-Optic Modulator EOM: Electro-Optic Modulator PCF: Photonic Crystal Fiber DG: Diffraction Grating PMT: Photomultiplier Tube

Fluctuation of light source and beat note fdye (a) frep frep time fAOM (b) frep time distribution of fbeat Frequency fdye + fAOM (c) frep frep Here, I explain the procedure how we determine the frequency axis. We assume the dye laser frequency increases with some noise. Then, we tune the frequency of the AO frequency shifter to cancel out the change of the laser frequency. So the sum of the laser frequency and the AO shift frequency becomes constant, but the deviation from the linear scan remains. When the shift frequency reaches at the operating limit, we reset the shift frequency by frep. So, if we monitor the beat frequency between the dye laser and the comb, the beat frequency shows almost constant value, but we can detect the deviation from the linear scan. For example, if the dye laser scan is linear but with Gaussian noise, the histogram of the beat frequency shows the Gaussian distribution. frep 40.6 41.0 fbeat [MHz] time fbeat (d) time

Line broadening due to IVR Excess energy 1560 cm-1 First, we show this transition. The excess energy is one hundred five hundred sixty wavenumbers. 2260 cm-1 2545 cm-1

Naphthalene spectrum: Ex = 1560 cm-1 1.16 fAOM [GHz] 1.00 fbeat [MHz] 41.2 40.6 I2 signal intensity [arb.unit] 16788.95 16789.0 Wavenumber [cm-1] 33577.9 33578.0 Wavenumber [cm-1] C10H8 signal intensity [arb.unit] 1818 1717 1616 1515 1414 1313 1212 1111 1010 99 88 77 66 55 44 33 Ka = 0 This is the shift frequency by AO frequency shifter. And this is the beat frequency between the dye laser and the comb. The beat frequency distribution can be fitted by the Gaussian function well. This is the saturation spectrum of iodine molecule. We use this iodine spectrum to confirm the absolute value of the frequency. This part is the Doppler-Free Two-Photon spectrum of naphthalene. Rotational lines are resolved well. Based on the regularity of the spectrum, we assigned these rotational lines. 1818 1716 1717 1615 1616 1514 1515 1413 1414 1312 1313 1211 1212 1110 1111 109 1010 98 99 87 88 76 77 66 65 54 55 43 44 32 33 Ka = 1 1716 1614 1615 1513 1514 1412 1413 1311 1312 1210 1211 119 1110 108 109 97 98 86 87 75 76 65 64 54 53 43 42 Ka = 2 Ka = 3 41 52 30 53 63 64 74 75 85 86 96 97 107 108 118 119 129 1210 1310 1311 1411 1412 1512 1513 1613 1614 42 31 1613 1612 1512 1511 1411 1410 1310 139 129 128 118 117 107 106 96 95 85 84 74 73 63 62 52 51 41 40 Ka = 4 1511 1510 1410 149 139 138 128 127 117 116 106 105 95 94 84 83 73 72 62 61 51 50 Ka = 5 149 148 138 137 127 126 116 115 105 104 94 93 83 82 72 71 61 60 Ka = 6 Ka J Kc 126 125 115 114 104 103 93 92 82 81 71 70 Ka = 7 103 102 92 91 81 80 Ka = 8

Line broadening due to IVR Excess energy 1560 cm-1 Next, we show this transition. The excess energy is two thousand two hundred sixty wavenumbers. 2260 cm-1 2545 cm-1

Naphthalene spectrum: Ex = 2260 cm-1 1.16 fAOM [GHz] 1.00 fbeat [MHz] 42.0 39.0 I2 signal intensity [arb.unit] 17139.29 17139.31 C10H8 signal intensity [arb.unit] This is a part of the observed transition. This is the shift frequency by AO frequency shifter. And this is the beat frequency between the dye laser and the comb. In this area, there is no iodine signal. And this is the Doppler-Free Two-Photon absorption spectrum of naphthalene. As you see, the rotational lines are resolved well. But, remember, in this old spectra, the lines started to become broad. However, in our spectrum, the lines are still narrow. So, we enlarge this line. 34278.58 34278.62 Wavenumber [cm-1]

Linewidth (J = 14, Ka = 0,1, Kc = 14) 2260 cm-1 1560 cm-1 Signal intensity [arb. unit] Signal intensity [arb. unit] 2.46 [MHz] 2.48 [MHz] 20 [Pa] 34278.585 .586 33577.915 .916 Wavenumber [cm-1] Wavenumber [cm-1] This is the enlarged view of one of the rovibronic lines in this transition. The full width at half maximum is 2.46 MHz. And this one is the enlarged view of one of the rovibronic lines in the lower transition. The full width at half maximum is 2.48 MHz. As you see, these linewidths have almost same value. So, just comparing these two lines, we cannot conclude IVR starts in this band. However, we have not assigned the rovibronic lines in this band. We will assign the rovibronic lines, and we will analyze the rotational dependences of the linewidth. Transit time broadening: Pressure broadening: Dye laser linewidth: 640 kHz (Beam diameter: 0.25 mm) 530 kHz for 20 Pa (190 kHz for 0 Pa (extrapolated value)) 210 kHz (~ ms), several tens of kHz (~ s) Natural width: 910 kHz (by lifetime measurement (175 ns))

Summary Summary Future plan Doppler-free two-photon absorption spectroscopy of naphthalene S11B1u (n4=1) ← S01Ag (n=0) band (Ex = 1560 cm-1) S11B1u (n4=1, n8=1) ← S01Ag (n=0) band (Ex = 2260 cm-1) was observed with an optical frequency comb as the frequency reference. In the obtained spectra, the rovibronic lines are well resolved. For above two bands, the linewidth of rovibronic lines are same. This is the summary. We observe the Doppler-Free Two-Photon absorption of naphthalene, these two bands. In this measurement, we use an optical frequency comb as a frequency reference. In the observed spectra, the rovibronic lines are well resolved. For these two bands, the linewidth of rovibronic lines are almost same. And our plan is, First, we will assign the rotational lines in this band, and we will study J, K-dependences of the linewidths. We will observe the higher vibronic band, assign the rovibronic lines, and analyze the linewidth. That's all. Thank you for your attention. Future plan We will assign the rotational lines in S11B1u (n4=1, n8=1) ← S01Ag (n=0) band, and will study J, K-dependences of the linewidths. We will observe S11B1u (n4=1, n7=1) ← S01Ag (n=0) band (Ex = 2545 cm-1).

Intramolecular vibrational energy redistribution (IVR) Intensity Borrowing Diagonalize System state For naphthalene, the transition to the S1 state is forbidden. But some vibronic states in the S1 state are allowed through intensity borrowing from the S2 state. Behind the system states, there are many bath states. Transition to the bath states are still forbidden. System states are coupled with the bath states. For higher excess energy, the density of the bath states become high, then the transition to the bath states increase. For benzene, clear dependences on J and K are known. But for naphthalene, do not. We expect such kind of J, K-dependence is exist also for naphthalene even if it is weak. Bath states Eigen states S1 S0

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 The sample cell is set in the Fabry-Perot resonator. Now, consider a molecule which travels at a velocity u along the laser beam. The absolute values of the Doppler shift of these two laser beams are same, but the sign is opposite. So, if the molecule absorbs one photon of each, the Doppler shift cancels out. This method also has other merits.

fdye, fAOM, fbeat fdye + 2fAOM scan fixed during a single step fbeat In the measurement, there are four cases depending on the direction of the change of the dye laser frequency and on the relative position of the dye laser frequency to the nearest comb mode. In a single step in our measurement, the shift frequency by AOM, fAOM, is kept constant. The observed beat frequency, fbeat, is the average in the single measurement step. For the first case, fbeat increases in the step, so the obtained fbeat becomes large value. And the second case, fbeat decreases in the step, so the obtained fbeat becomes small value. fbeat Optical frequency

Naphthalene spectrum: Ex = 2260 cm-1 1.16 fAOM [GHz] 1.00 fbeat [MHz] 42.0 39.0 I2 signal intensity [arb.unit] 17139.0 17139.7 C10H8 signal intensity [arb.unit] This is the wide range spectra for two thousand two hundred sixty cm-1 band. In this measurement, around here, the dye laser fluctuation amounts about 1 MHz. This value is too large for the present purpose, so we have to try again. But this also is the strong point of our method. We clearly understand the measurements around here are not satisfactory. 34278.0 34279.5 Wavenumber [cm-1]