Time-resolved Raman

Spectroscopy

2013.11.14

Carl-Zeiss Lecture 3

IPHT Jena

Hiro-o HAMAGUCHI

Department of Applied Chemistry and Institute of Molecular

Science, College of Science, National Chiao Tung University,

Taiwan

「諸行無常」 Nothing can last for ever

Time and Space: When？ Where？

Sun set at Shinjiko Lake. http://blogs.yahoo.co.jp/naru3075/41266684.html

Organ

Cell

Organelle

Liposome

Solvation structure

Complex

Aggregate

Molecule

Atom

Biology

Chemistry

Physics

Time Space

109 s

103 s

102 s

10-10 s

10-12 s

10-15 s

10 cm

10 mm

1 mm

10 nm

1 nm

0.1 nm

Ionic liquid

Structures of Diphenylacetylene in Different Electronic States

2099 cm-1

1567 cm-1

2223 cm-1

1974 cm-1

1500 1000 500

Wavenumber / cm-1

0min

11min

31min

41min

62min

72min

6min

24min

53min

69min

Time-resolved Raman Spectroscopy of a Dividing Yeast

Y-S Huang, T. Karashima, M. Yamamoto, H. Hamaguchi, J. Raman Spectrosc., 34, 1-3 (2003).

http://www.ed.kanazawa-u.ac.jp/~kashida/ChemII/chapter1/sec1/c112.htm

How Does Chemical Reaction Proceed?

When？ Where？

Photophysics and Photochemistry of Molecules

S0

S1

S2

T1 Flu

ore

scen

ce

Ph

osp

horescen

c

Chemical

Reactions

Optical transition Vibrational relaxation Internal conversion Intersystem crossing

Ph

oto

excita

tion

Nanosecond Transient Raman Spectrometer (1983)

O OH

OH

H

3

O

*

2

HO OH

Benzophenonein the T1 state

(T1-BP)

h ISC

Benzopinacole

Benzophenone(BP)

Benzophenone ketyl radical

(BPK)

Benzhydrol(BH)

H-atom abstraction

Photochemical Hydrogen Abstraction Reaction of Benzophenone

T1

S1

S0

Probe: 532 nm

5 ns duration

15 ns delay

Pump: 266 nm

5 ns duration

Intersystem

crossing

Tn

Nanosecond Time-resolved Raman Spectroscopy of BP in CCl4

Pump + Probe

Probe only

T. Tahara, H. Hamaguchi and M. Tasumi, JPC (1987).

Fig.5

h1013C

d513C

h10

d10

d5

T1 Raman Spectra of Isotopically Substituted Benzophenones

O OH

OH

H

3

O

*

2

HO OH

Benzophenonein the T1 state

(T1-BP)

h ISC

Benzopinacole

Benzophenone(BP)

Benzophenone ketyl radical

(BPK)

Benzhydrol(BH)

H-atom abstraction

CO stretch

Raman Spectrum and Structure of T1 Benzophenone

1400 1800 1000 600

Raman Shift / cm-1

T1 state

S0 state

16

58

cm

-1

12

22

cm

-1

Photoisomerization of Trans-Stilbene

Probe solvent dependent structure and dynamics of S1 trans-stilbene by time-

resolved Raman spectroscopy

h Solvent-dependent isomerization rate

Hexane 70 ps

Dodecane 120 ps

Why, when and how rotation occurs in the excited state?

Nanosecond Transient Raman Spectrum

of S1 Trans-Stilbene (1983)

H. Hamaguchi, C. Kato, M. Tasumi, Chem. Phys. Lett., 100, 3-7 (1983).

Iwata, Yamaguchi, and Hamaguchi, Rev. Sci. Instrum. 64 , 2140 (1993).

cw Nd:YAG regenerative amplifier

cw mode-locked Nd:YAG laser

pulse compressor dye laser

SHG

optical delay

sample

multichannel detector (CCD)

pump

probe

SHG

polychromator

sync. pumped

band-reject filter

variable ND

UV cut filter

dichroic mirror

dichroic mirror

SHG

PD

82 MHz 588 nm

amplifier

dye AO

S0

Sn

S1 294 nm 2.2 ps

294 nm 2.2 ps

3.2 ps 588 nm

3.5 cm-1

3.2 ps 588 nm

3.5 cm-1

Trans-Stilbene

Picosecond Transform-limited Time-resolved Raman Spectrometer

Picosecond Transform-limited Time-resolved Raman

Spectroscopy

10

8

6

4

2

0

/

cm

-1

1086420

t / ps

Obs

(t = 3.2 ps, = 3.5 cm-1

)

Accurate peak position

and band shape

Picosecond Time-resolved Raman Spectra of S1 trans-Stilbene in CHCl3

1700 1600 1500 1400 1300 1200 1100

Raman Shift / cm-1

2 ps

20 ps

100 ps

-1 ps

0 ps

10 ps

7 ps

3 ps

30 ps

70 ps

-2 ps

50 ps

5 ps

1 ps

15 ps

40 ps

-10 ps Pump 294 nm

Probe 588 nm (0.1 mW)

C=C stretch vibration

1560 cm-1: double bond

Why rotation occurs

around a double bond ?

1620 1600 1580 1560 1540 1520

Raman Shift / cm-1

Hexane

Decane

Nonane

Octane

Heptane

Hexadecane

Dodecane

Fig. 7 H. Hamaguchi and K. Iwata

The C=C Stretch Raman Band of S1 trans-Stilbene in Alkanes

The peak position shifts to higher wavenumbers and the band width decreases

on going from hexane to hexadecane. Why?

1574

1572

1570

1568

Peak p

ositio

n /

cm

-1

22 21 20 19 18 17

Bandwidth / cm -1

hexane

octane

decane

Peak Position vs Band Width of the C=C Stretch Raman Band of S1 trans-

Stilbene In Alakne Solvents at Different Temperatures

Why does linear

Relation hold?

1578

1576

1574

1572

1570

1568

1566

Peak p

ositio

n /

cm

-1

20x10 9

15 10 5 0 k iso

/ s -1

283 K

293 K

298 K

303 K

313 K

Peak Position of the C=C Stretch Raman Band vs the

Isomerization Rate of S1 trans-Stilbene

Why does linear

relation hold ?

Why do lines

converge ?

気相

O N

C H

時間 /ps

双極子モーメント

/ D

Theory of Vibrational Dephasing and Band Shape

Simulation of Acetone/Acetonitrile system

Vibrational frequency is stochastically modulated in solution

Solvent-

induced

dynamic

polarization

Solvent-induced Dynamic Polarization of Acetone in

Acetonitrile

Formulation of Vibrational Band Shapes under Dynamic

Polarization

1

2

1

2

Modeling

-110 M: 2.94×

: 2.04 10-2

M×

: 1.35M: 2.29M

1. 0

0. 8

0. 6

0. 4

0. 2

0. 0

1000990980970960 1010

Raman shift / cm-1

inte

nsity

/arb

.unit

1 2

t =

(1-2)/W2

C=C

C+-C-

W 1

W 2

W 1

W 2

1 2

(c) Continuous Frequency Modulation Model

(b) Many Frequency Exchange Model

(a) Two Frequency Exchange Model

W = W1t/(1+t2)

G = W1t2/(1+t2)

G / W = t

),1/()/(2

1

2

11

tt +=W =

WWWn

),1/()/(22

1

2

11

tt +=G =

WWWn

+

=W0

02

1

)(

)1/()(

tt

tttt

dG

dGW

+

=G0

022

1

)(

)1/()(

tt

tttt

dG

dGW

G / W = t1/2 ； G(t)=exp(-ln2t2/t1/22)

Extension of the Two Frequency Exchange Model

Hamaguchi Mol. Phys.89, 463 (1997).

1574

1572

1570

1568

Peak p

ositio

n /

cm

-1

22 21 20 19 18 17

Bandwidth / cm -1

hexane

octane

decane

Peak Position vs Band Width of the C=C Stretch Raman Band of

S1 trans-Stilbene In Alakne Solvents at Different Temperatures

W = W1t/(1+t2)

G = W1t2/(1+t2)

G / W = t

t =1.4

1578

1576

1574

1572

1570

1568

1566

Peak p

ositio

n /

cm

-1

20x10 9

15 10 5 0 k iso

/ s -1

283 K

293 K

298 K

303 K

313 K

Peak Position of the C=C Stretch Raman Band vs the

Isomerization Rate of S1 trans-Stilbene

1=1577.3 cm-1

0

1

2

3

4

5

W

/ c

m-1

20x10 9

15 10 5 0 k iso

/ s -1

283 K

293 K

298 K

303 K

313 K

Peak Position of the C=C Stretch Raman Band vs the

Isomerization Rate of S1 trans-Stilbene

1x1012

W1

/ s-1

W = W1t/(1+t2)

t =1.4

W =0

0

1

2

3

4

5

W

/ c

m-1

20x10 9

15 10 5 0 k iso

/ s -1

283 K

293 K

298 K

303 K

313 K

Peak Position of the C=C Stretch Raman Band vs the

Isomerization Rate of S1 trans-Stilbene

1x1012

W1

/ s-1

kiso=

W1P(T)

1640 1600 1560 1520 1480

Raman shift / cm-1

Dodecane

Hexane Obs Fit

Fig. 10 H. Hamaguchi and K. Iwata

Fitting of the observed band shapes

Fitting of the C=C Stretch Raman Band of S1 trans-

Stilbene by the Two Frequency Exchange Model

W1=2.7x1012 sec-1 (370 (fs)-1)

W1=1.5x1012 sec-1 (670 (fs)-1)

W 1

W 2

W 1

W 2

W = W1t/(1+t2)

isomerization rate

Hexane 70 ps

Dodecane 120 ps

Isomeri-

zation

W1

W2

Dynamic Polarization Model of Isomerization

Hamaguchi, Iwata, CPL 208, 465 (1993).

Deckert, Iwata, Hamaguchi, J. Photochem. Photobiol. 102, 35 (1996).

Iwata, Ozawa, Hamaguchi, JCP 106, 3614 (2002).

kiso=W1P(T): Dynamic Polarization Model

E=3.5~3.7 kcal mol-1 (Raman band shape)

A new view on isomerization has come out of picosecond Raman spectroscopy !

kiso=Aexp(-E/RT) : Arrhenius formula

E=3.5 kcal mol-1 (fluorecsence lifetime)

kiso= W1P(T)

What Are Ionic Liquids？ Liquids that are composed solely of ions

Why are they liquids? Green Solvents?

Liquid Structures?

Ionic Liquids

Crystal Liquid Crystal Liquid

Known condensed phases of matter

Are Ionic Liquids Really Liquids in the Conventional Sense?

Transparent but heterogeneous fluid

with well-defined local structures ?

Ionic Liquid ?

Picosecond Energy Dissipation Dynamics

of Photoexcited S1 Trans-stilbene

at 50 ps

Picosecond Raman Thermometer with S1 trans-

stilbene

K. Iwata and H. Hamaguchi, J. Phys. Chem. A, 101, 632 (1997)

Peak position

Temperature

Transient Raman spectra

S1 trans-stilbene at 50 ps

Raman

thermometer

pump 294 nm probe 588 nm C＝C str.

High

temperature

Low

temperautre

37

Vibrational Cooling Process of

S１ Trans-stilbene in Decane

Peak shift to higher wavenumber

C=C str.

= 0.10 ps-1

pump 296 nm probe 592 nm

Time-resolved

Raman spectra

cooling

rate k

vibrational

cooling process

Cooling process

38

Correlation between

cooling rate and thermal diffusivity

K. Iwata and H. Hamaguchi, J. Phys. Chem. A, 101, 632 (1997).

Cooling rate

(Ultrafast dynamics)

Thermal diffusivity

(Bulk property)

Much faster cooling

rates in ionic liquids

Cooling Rate vs Thermal Diffusivity

Macroscopic and Microscopic Thermal Diffusion in

Ionic Liquids

Cooling rate

(microscopic)

Thermal diffusivity

(macroscopic)

between molecules

between

”Local Structures”

Absorption Spectrum of trans-stilbene in C2mimTf2N

2500 cm-1

① 325 nm excitation； No excess energy ② 297 nm excitation； Excess energy 2500 cm-1

②

①

Cooling Kinetics of S1trans-stilbene in C2mimTf2N

Coherent artifact

Simulation of Heat Dissipation in an Ionic Liquid

Tr.t. =T

2{ [erf{(a /2 + nL) (4Dtht)

1 2} +erf{(a /2 nL) (4Dtht)1 2}]

n =-

}

{ [erf{(b /2 + nL) (4Dtht)1 2} +erf{(b /2 nL) (4Dtht)

1 2}]n =-

}

{ [erf{(c /2 + nL) (4Dtht)1 2} +erf{(c /2 nL) (4Dtht)

1 2}]n =-

}

Temperature at the heat origin

a, b, c : Size of the

heat origin

L: Size of Local Structure

K. Iwata and H. Hamaguchi, J. Phys. Chem. A, 101, 632 (1997)

Heat origin:

Hot S1 trans-stilbene + solvent

Hot S1 trans-stilbene

Heat origin Local structure

Simulation of the Cooling Kinetics

Size of local structure > 10 nm

Photophysics ; closely lying excited states (S3, S2, S1, T1)

Photochemistry; photoisomerization

・Asymmeric trans-cis photoisomerization ・Marked solvent effect

Photobiology; retinoid proteins

・Rhodopsin ・Bcteriorhodopsin ・Sensoryrhodopsin

O 9-cis all-trans

Retinal: a prototype molecular system for

efficient

inefficient

O

Photoisomerization of All-trans Retinal

O

non-polar solvent

polar solvent Why?

O

O

O

O

O

13-cis

7-cis

11-cis

7 A ll-trans

9-cis

9 11 13 15

Proton Pumping by Bacteriorhodopsin

http://anx12.bio.uci.edu

All-trans isomerizes to 13-cis exclusively. Why and How?

9-cis All-trans

S3

S2

S1

S0

T1

O

O

What happens in the Excited electronic

States of Retinal ？

h h

Nanosecond Pump/Probe Time-resolved Raman

Spectroscopy of Retinal Isomers

H. Hamaguchi, H. Okamoto,and M. Tasumi, Chem. Lett. 1984, 549-550.

H. Hamaguchi, H. Okamoto, M. Tasumi, Y. Mukai, and Y. Koyama,

Chem. Phys. Lett. 107, 355-359 (1984).

Tn

T1

S1

S0

Probe: 532 nm

5 ns duration

15 ns delay

Pump: 355 nm

5 ns duration

Intersystem

crossing

Raman

Scattering

Raman Spectra of All-trans- and 9-cis-Retinal in the T1 and S0 States

All-trans-retinal

T1 State S0 State

9-cis-retinal

T1 State S0 State

9-cis All-trans

S3

S2

S1

S0

T1

O

O

15 ns after

excitation

One-way Photoisomerization on the T1 surface

h h

10 Picosecond Time-frequency Two-dimensional

Multiplex CARS Spectroscopy of Retinal Isomers

Tn

T1

S1

S0

Pump: 400 nm

500 fs duration

Intersystem

crossing

T. Tahara, B. N. Toleutaev, and H. Hamaguchi, J. Chem. Phys. 217, 369-374 (1994).

T. Tahara and H. Hamaguchi, Rev. Sci. Instru. 65, 3332-3338 (1994).

1 2

1

21-2

Probe:

1 523 nm, 5 ns

2 broadband, 5ns

Experimental Setup for 2-D CARS Spectroscopy

Wavenumber range: ~600 cm-1

Time-resolution: 15 ps

Picosecond Two-dimensional Multiplex CARS Spectroscopy

(2-D CARS)

http://cvitae.org/content/view/180/895/

Operation Principle of a Streak Camera

2-D CARS Image for All-trans-retinal in

Cyclohexane

Wavenumber / cm-1 Wavenumber / cm-1

Picosecond Time-resolved CARS Spectra of Retinal Isomers

in Cyclohexane

All-trans 9-cis

Population Rise of the All-trans T1 State Monitored by CARS

890 ps

9-cis All-trans

S3

S2

S1

S0

T1

O

O

Photoisomerization on the Triplet Potential Surface

900 ps

Asymmetric: cis

to trans only h h

Nanosecond Pump/Probe Time-resolved Infrared

Absorption Spectroscopy of Retinal Isomers

Tn

T1

S1

S0

Pump: 347 nm

5 ns duration

Intersystem

crossing

T. Yuzawa and H. Hamaguchi, J. Mol. Struct., 352, 489-495.

T. Yuzawa and H. Hamaguchi, in preparation.

Probe:

cw infrared continuum

Nanosecond Time-resolved Infrared Absorption Spectrometer

4000 cm-1 ~ 700 cm-1

50 ns time resolution

Time-resolved Infrared Absorption Spectra of Photoexcited

All-trans-retinal in Cyclohexane

Singular Value Decomposition (SVD) Analysis

SVD Analysis of the Time-resolved Infrared Absorption Spectra of All-trans Retinal

Spectral component Singular Values Temporal component

SVD Analysis of the Time-resolved Infrared Absorption Spectra

of All-trans Retinal

1.0

0.5

0.0

Am

plit

ud

e

403020100

Time / ms

1.0

0.5

0.0

Am

plit

ud

e

403020100

Time / ms

all-trans S0

h

all-trans S1

all-trans T1

13-cis S0 9-cis S0

ISO = 0.27 ~0.43

3 : 1

ISC = 0.43~0.7

QY_estimation4

The schematic diagram of the photoisomerization mechanism of all- trans-retinal

ISO =

[photoexcited all-trans S0]

[13-cis S0 + 9-cis S0]

(calculated)

(literature)

photoisomerization via the singlet excited state

Conclusion 1

Photoisomerization Pathway of All-trans-retinal in

Cyclohexane

9-cis All-trans

S3

S2

S1

S0

T1

O

O

5 ms << 50 ns

Two Relaxation Pathway of the Photoexcited All-trans-retinal

h

Femtosecond Pump/Probe Time-resolved Visible and

Ultraviolet Absorption Spectroscopy of Retinal Isomers

Tn

T1

S1

S0

Intersystem

crossing

S. Yamaguchi and H. Hamaguchi, J. Mol. Struct. 379, 87-92 (1996).

S. Yamaguchi and H. Hamaguchi, J. Chem. Phys. 109, 1397-1408 (1998).

H. Minami and H. Hamaguchi, to be published.

S2

S3

Probe:

Femtosecond

White Cntinuum

Pump:

400 nm

50 fs duration

Femtosecond Time-resolved Visible/Ultraviolet Absorption System

Chirp Correction for the Observed Femtosecond Time-

resolved Absorption Spectra

Femtosecond Time-resolved Visible Absorption Spectra in Hexane

All-trans 9-cis

Wavelength / nm Wavelength / nm

Cascading Decay Kinetics of Photoexcited All-trans-retinal in

Heptane

Excitation: 400 nm

9-cis All-trans

S3

S2

S1

S0

T1

O

O

50 fs

400 fs

32 ps

34 ps

700 fs

50 fs

Femtosecond Photoexcitation Dynamics of Retinal Isomers

h h 5 ms

900 ps

Femtosecond Time-resolved Ultraviolet Absorption

Spectra of All-trans retinal in Hexane

Wavelength / nm

SVD Analysis of the Femtosecond Time-resolved Ultraviolet

Absorption Spectra of All-trans-retinal in Hexane

9-cis All-trans

S3

S2

S1

S0

T1

O

O

7 ps

Trans to Cis Isomerization via the S2 state

9-cis All-trans

S3

S2

S1

S0

T1

O

O

50 fs

400 fs

32 ps

34 ps

700 fs

50 fs

Photoisomerization Pathways and Dynamics of Retinal Isomers

h h 5 ms

900 ps 7 ps

Picosecond Time-resolved 2-D CARS Spectroscopy with an

Optical Kerr Gating

Streak camera

Ti:sapphire

oscillator / regenerative

amplifier system SHG

Optical delay

Spectro-

graph ns Nd:YLF

& dye laser

Sample Kerr cell

Analyzer Filter

Filter

waveplate Gating pulse

~800 nm

Polarizer

Excitation

pulse

~400 nm

Actinic light: 90°

1: 90°

2: 90°

Analyzer: 0°

Detector

Gate pulse: 45º

CARS signal: 90º

Kerr medium (CS2)

Optical Kerr Gating

Optical Kerr Gated Picosecond CARS

Spectroscopy

Picosecond Time-resolved CARS Spectra of

Photoexcited All-trans Retinal in Ethanol : Simulation

K. Ishii and H. Hamaguchi, Chem. Phys. Lett, 367, 672 (2003).

First Observation of the key intermediate S2

S0

S2

S0+T1

S3

trans

h

S0

T1

S1 S2

P*

P0

S0

cis

“perpendicular”

Large band width of the C=C band (100 cm-1)

Very fast vibrational dephasing

Dynamic polarization model of isomerization

0

・S2の振動スペクトル測定に初めて成功

・C=C伸縮バンドの顕著な広幅化(100cm-1)

異性化反応の動的分極理論と符合

S0

T1

S1

S3

S2

P*

P0

S0

trans cis

“perpendicular”

h

S0の減少

S２の生成、消滅

S0の回復

Cis体の生成

Ｔ１の生成

動的分極

9-cis All-trans

S3

S2

S1

S0

T1

O

O

Ns Raman

10ps CARS

Ns IR

Fs UV-VIS Ps CARS

Time-resolved Spectroscopies Look at Photoisomerization of

Retinal

Isomeri-

zation

W1

W2

Dynamic Polarization Model of Isomerization

Hamaguchi, Iwata, CPL 208, 465 (1993).

Deckert, Iwata, Hamaguchi, J. Photochem. Photobiol. 102, 35 (1996).

Iwata, Ozawa, Hamaguchi, JCP 106, 3614 (2002).

kiso=W1P(T): Dynamic Polarization Model

E=3.5~3.7 kcal mol-1 (Raman band shape)

A new view on isomerization has come out of picosecond Raman spectroscopy !

kiso=Aexp(-

E/RT) : Arrhenius formula

E=3.5 kcal mol-1 (fluorecsence lifetime)

kiso= W1P(T)

9

O 7 9 11

O

13 1 5

7 11 13

O

1 5

7 9 11 13 1 5

hyperconjugation

014

Dynamic Polarization Model for the Retinal

Photoisomerization

Why are 13-cis and 9-cis selected?

hyperconjugation

Absolute Temperature Determination

with Stokes/anti-Stokes Raman spectroscopy

Temperature

at the laser focal spot?

Stokes/anti-Stokes Raman Scattering

|v=0>

|v=1>

Stokes antiStokes

Boltzmann factor

Off-resonance Raman scattering

Sensitivity Calibration with Standard Light

Rotational Raman spectra as primary intensity standard

[1] H. Hamaguchi, I. Harada, T. Shimanouchi, Chem. Lett. 1974, 12, 1405-1410.

Previous study [1]

Sensitivity calibration with D2

D2[1]

Accuracy of the

Standard Intensity

Accuracy of

Sensitivity Calibration

This study

Sensitivity calibration with N2

-200 ~ +200 cm-1, 40 lines

Suitable for low-frequency region

1.0

0.8

0.6

0.4

0.2

0.0

cross

-sec

tion r

atio

2015105

Initial J

Calculated Raman Cross Section Ratios of N2

Intensity Standard

cross-section

Low Frequency Raman Microspectrometer

Spectral resolution: 1 cm-1

Nd:YVO4

Laser (532 nm)

Sample

Polarizer

Brag Notch filters

Spectrometer

f=50 cm

1800 gr/mm

Beam

sampler

Objective (x20

NA0.45)

Pinhole

(75 um)

CCD

Long pass

filter

Halogen lamp

Raman

scattering

Optical

image

Dichroic

mirror

Rotational Raman Spectrum of N2

532 nm, 20 mW, x 20 Objective,

exposure: 30 min, resolution: 1.3 cm-1

Background (glass, air) was subtracted

0

Inte

nsi

ty /

a.u

.

200 100 0 -100 -200

Raman shift / cm-1

Trace Naphthalene Melting

Spectral resolution: 1 cm-1

Peltier

heat/cool stage

Thermocouple

(on the stage)

Sample

Naphthalene

Mp. 353 K (80 °C)

532 nm, 4 mW, x 20 Objective

Exposure: 0.1 sec

Cycle: 0.22 sec / spectrum

Heat speed: 20 K / min

(on the heat stage)

Low-frequency Raman Spectrum of Naphthalene

532 nm, 4 mW, x 20 Objective,

exposure: 80 sec

Temperature determination

(-30 to -170 cm-1)

solid

400 200 0 -200Raman shift / cm -1

(b)

174.6 s

191.4 s

192.7 s

100 μm

420

400

380

360

340

320

300

280

Tem

per

atu

re /

K

200150100500Heat time / sec

m.p.

Heating

(a)

Solid

Liquid

2)

plateau

(50~120

s)

3)

fluctuation

(120 ~ 192 s)

> 50

K

4) melting

(192.7 s~)

Unusual Thermal Behavior before Melting

0) at r.t.

(< 0 s)

1)

increasing

to m.p.

(0~50 s) Local temperature fluctuation just before melting?