1. 2 Univ. of Electro-Communications
High Resolution Spectroscopy of A 1B1u <- X 1Ag 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

2. 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

3. 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).

4. 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

5. 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

6. 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

7. 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

8. 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 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

9. 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
fbeat [MHz]
time
fbeat
(d)
time

10. 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

11. Naphthalene spectrum: Ex = 1560 cm-1
1.16
fAOM
[GHz]
1.00
fbeat
[MHz]
41.2
40.6 I2
signal intensity
[arb.unit]
Wavenumber [cm-1]
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

12. 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

13. Naphthalene spectrum: Ex = 2260 cm-1
1.16
fAOM
[GHz]
1.00
fbeat
[MHz]
42.0
39.0 I2
signal intensity
[arb.unit]
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.
Wavenumber [cm-1]

14. 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]
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))

15. 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).

16. 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

17. 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.

18. 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

19. Naphthalene spectrum: Ex = 2260 cm-1
1.16
fAOM
[GHz]
1.00
fbeat
[MHz]
42.0
39.0 I2
signal intensity
[arb.unit]
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.
Wavenumber [cm-1]