Case study of laser wave packet measurement in Shenzhen Double tenth

Date:2022/6/23 9:17:08 / Read: / Source:本站

A case study on laser wave packet measurement in Shenzhen Double 10 analyzes the photon absorption, while the molecule is excited to a specific vibrational level in a higher electron excited state, the process depends on  

The Franck-Condon transition principle.  

The transition state was detected by fluorescence (et)  

For 12, this higher excited state is a fluorescence state, so each delay can be detected in the experiment  

The variation of laser induced fluorescence intensity with delay time.  Because in one period of vibration, the atoms leave first  

And finally return to its starting position, and the laser induced fluorescence intensity also changes correspondingly, and finally  

We get the change in R over time.  If the frame vibration is harmonic, only a single vibration cycle can be observed  

, as shown in Figure 1.16.  If the vibration is anharmonic, the vibration period is related to the energy of the oscillator, resulting in packets  

The superposition state with multiple vibration periods is the wave packet.  If the delay detection pulse is able to  

Bam floU  

12, fluorescence measurement can be changed to ion imaging or mass spectrometry to measure the content of I}z,  

The dynamic information of wave packet evolution is also given.  

Figure 1.17. Rotational initial phase, half-phase recovery and total phase return of a ensemble consisting of three linear molecules as ideal rotors  

Complex diagram  

The angles contained in the curved arrows show how much the molecules are rotating at each time step, and the three molecules are rotating at different speeds.  Worth pointing out  

Yes, the quantum theory of rotation shows that at the half-phase recovery point "1/ (4B)], the rotational orientation of the molecule is inverted with the initial position of 18O  

(Negative signal)  

Only those molecules in the ensemble whose transition moment is parallel or nearly parallel to the polarization plane of the excited photoelectric field E can be  

Effective excitation, because the amplitude of the rotational transition is small depending on 5"E.  It can be seen that the initial orientation of the ensemble is due to  

Caused by anisotropic excitation.  Polarization measurement of laser-induced fluorescence can reflect the orientation of ensemble molecules at any time  

The change.  In the evolution of ensemble rotation, the molecular rotation speed of high rotational quantum state J is faster than that of low rotational quantum state J  

And is accompanied by attenuation of the coherent states.  The wider the distribution of the rotational quantum state J formed during the excitation process, the phase  

The dry state also decays faster.  

Because the iodine molecule in the experiment is isolated, and free rotation, after a period of evolution of the molecular  

Orientation can be returned to its initial state (phase recovery).  If the centrifugal distortion effect of the molecule is ignored, then the phase  

The recovery time of is completely determined by the molecular rotation constant B of the particular vibrational energy level, namely the recovery time  

(rephasingTime) is 1/ (2B).  Therefore, for every excitation pulse preparation, there exists one  

Rotational phase recovery time.  FIG. 1.17 shows only the excitation of a single vibration.  In the actual process  

There are usually many vibrational levels excited at the same time.  In this case, when the rotational phase recovers  

The interval is determined by the rotational constant B values corresponding to different vibrational energy levels.  Thus, rotational motion can consist of two  

The dynamic detuning in dry period caused by the disappearance of initial orientation and after a long time interval  

Phase recovery due to molecular orientation recovery.  In order to eliminate the interference of molecular orientation in the experiment, we only need to excite  

The Angle between the polarization plane of the pulse laser and the probe laser is set to the magic Angle (54.70) (see Chapter 11).  

Figure 1.18 shows the transition state time-resolved laser-induced fluorescence after eliminating the effect of molecular orientation rotation  

Kinetic curve.  The most striking feature in the figure is the rapid oscillation with a period of 300fs, corresponding to the vibration cycle of the molecule  

While the amplitude changes much more slowly, and its modulation period is LOPS, this slow period modulation is caused by the vibration of non  

Caused by harmonic, indicating the interference between vibration dynamics of different frequencies in the wave packet.  The two main things about the Fourier transform  

The vibration mode can be compared with the high resolution spectral results of iodine molecules.  

Figure 1.18 Kinetics curves of transition state time-resolved laser-induced fluorescence after eliminating molecular orientation rotation effect  

(a) Real-time vibration of iodine molecules shown by femtosecond time-resolved transition state spectroscopy (left), with two vibration periods of 3OOFS and LOPs respectively.  longitudinal  

The coordinates are the amplitude of laser-induced fluorescence, which can be used as a scale of atomic spacing.  The corresponding Fourier transform spectrum at 3.3thz is shown on the right  

There are two main vibration modes with o. Ithz (3cm-1) intervals near the wave number.  (b) Theoretical calculation results, as shown in Figure (a) 

The wave packet containing two main vibration periods and the corresponding Fourier transform spectrum  

It is possible to measure the rotational motion of ensemble molecules in real time if polarization measurement is used in the experiment.  figure  

1.19 is the polarization femtosecond time-resolved laser-induced fluorescence curve of iodine molecule rotation observed in the experiment, and  

The only difference in FIG. 1.18 is the Angle between the polarization plane of the excitation light and the detection light, which is measured in parallel and vertical directions respectively  

The amount.  The vanishing process of the spatial orientation of the early ensemble is clearly shown in the figure [Figure 1.19 (a), left, envelope  

Indicating part], where the fast oscillation part is the coherent excitation of vibration dynamics with a period of 3ooFS.  Parallel and vertical  

The amplitude of the directional measurement is significantly different at the beginning, and tends to be constant after about 3Ps, indicating that the spatial orientation of the partition is complete  

All disappear.  After a considerable period of time (about 6oops), there is a complete recovery letter in the integrated rotational phase  

Corresponds to the phase recovery process of rotational wave packet formed by the superposition of six oscillations (quantum number 7"12).  

Figure 1.19 Polarization femtosecond time-resolved laser induced fluorescence kinetics of iodine molecule  

(a) Transient spectra reveal the disappearance of molecular orientation in the initial ensemble due to molecular rotation (left, indicating part of the envelope line, fast oscillation is the vibration dynamic  

Coherent excitation) and complete recovery of the molecular orientation after a period of time (6oops) (right). Polarization parallelism of the probe light is shown respectively  

And perpendicular to the excitation light, the phase difference between them is 18o0, which is consistent with the theoretical prediction.  <b) molecular rotational detuning (left) and phase return  

Complex (right) theoretical calculation results.  It is assumed that the coherent states cover the six oscillations of the quantum number right 7"'12, and the vibration-transformation synthesis is considered  

Centrifugal distortion effect  

As expected by theory, the oscillating signals of rotational wave packets detected in parallel and vertical polarization phase differ  

1800.  The half-periodic rephasing expected by theory at 1/ (4B) was observed experimentally.  

Signals (not shown in Figure 1.19).  It can be seen that the vibration and rotation of the wave packet motion is very separate in time, oscillation period  

30OFS and 6OOPSO, respectively  

(2) Experimental observation of fractional-periodic phase recovery of diatomic Br2 vibrational wave packet  

From the theoretical analysis of the wave packet above, it can be seen that there exists a phase recovery period for anharmonic oscillators  

Trev-27i/ (B6)), and there is no super-recovery period.  So when t>Trev, the wave function will reproduce the wave many times  

The initial state of the package.  For example, whenever t is an integer multiple of Trev, the phase is also an integer multiple of 27T, and the wave packet returns to the beginning  

State.  At some special moment, when t/Trev= P /n is irreducible fractional rational number, the wave packet converges into a series  

Wavelet is called fractional - period recovery wave packet.  The motion of wavelet packet in fractional period also has periodicity  

The period was scores multiple of Te.  In the experiment, 1% OF Br2 was diluted into helium, and the pulse width was smaller than the roofs and the wavelength was 56onm  

The ground state olfactory molecules were excited to B state by a pulsed laser of the same pulse width and wavelength of 29onm  

Light maps the wave packet directly to the ionized state, that is, by absorption of two photons at 290nm.  Formation of Br.  With the quality  

Spectrometer detection.  The experimental results are shown in Figure 1.20.  

Figure 1.20 Br given by pump-i probe scan experiment;  A signal whose strength varies with time  

The curve below is the result of experimental measurement;  The curve above is the result of theoretical calculation.  The high frequency vibration is caused by fractional wave packet recovery  

P / 4 = 1/4, 1/2...  So marked  

The initial stage of wave packet dynamics has several vibration peaks, corresponding to the classical granular vibration with Tc= 3ooFS  

After L cycles, due to phase detuning between the coherent states of the wave packet, the vibration peak disappears until 6-  

Within the delay period of 9ps, the classical vibration cycle reappears, and this wave packet recurrence process corresponds to the half-recovery cycle  

Phase (1/2Trev), this process occurs every 8ps, so it can be determined that the recovery period Trev=16ps, the number  

And the theoretical expected value Trev=27t/ derived from the Mores potential approximation of the excited state of olfactory molecule B  

(Bo) consistent [12].  In addition, in the figure, during the non-classical grain period, i.e. T =1/4 trev,  

T = 3/4 trev, t = 5/4 trev,...  In which the frequency of vibration is twice that of the classical particle.  

These moments correspond to the fact that the wave packet is split into the same two halves, the phase is opposite, and the vibration period is the classical particle period  

In the half.  Figure 1.21 shows a theoretical simulation based on an accurate Ryderg-Klein-Rees (RKR) potential function  

Wave packet evolutionary dynamics curve, the corresponding p/ Q position is marked in the graph.  

The initial wave packet is generated by the pumped laser at the internal inflection point of the upper energy situation energy curve (a). The wave packet deharmonizes rapidly and forms a diffuse wave packet  

(b).  The coherence is partially restored at the 1/4 periodic recovery point (c), where the wave packet is split into two wavelet packets, located respectively at the potential curve  

Inside and outside point of inflection.  The wave packet with complete coherence recovery, such as the 1/2 periodic recovery, is located outside the outward-return point of the potential energy curve,  

The shape is very close to the initial wave packet.  In (c), the wave packet at the 1/4 cycle recovery can be regarded as the superposition of wave packet (a) and wave packet (d), two subsets  

The wave packet moves in the opposite direction, resulting in the opposite phase, and the motion frequency of the total wave packet is 2CJc  

The theoretical simulation results of four wave packet shapes at different time}q' (X, t) 12 shown in FIG. 1.21 further reveal waves  

The score cycle recovery process for packages.  The initial position of the wave packet at time zero is located at the internal inflection point of the potential energy curve,  

And vibrate in classical particle mode at frequency 6)C in the potential well [FIG. 1.21 (a)].  After a period of time, the wave packet  

In the state of complete detuning [FIG. 1.21 (b)].  FIG. 1.21 (C) shows wave packet in 1/4 fractional periodic recovery state, wave  

The packet consists of two identical wavelet packets in opposite phase, each of which vibrates at We frequency, giving the total vibration  

The dynamic frequency is 2WC.  FIG. 1.21 (d) shows the junction of a half-periodic recovery wave packet appearing for the first time at the outward-inflection point  

Structure, the wave baud over the inner point of the distressed rate (.)  C Classical particle - R mode vibration.  

(3) Measurement x of picmeter-scale ultra-fast wave-packet interference quantum fringe [13]  

The time evolution of a weakly anharmonic system when several quantum states are excited by resonance  

It is the wave packet recovery phenomenon, that is, a spatially well localized wave packet first disperses and then goes through a period of precise recovery  

After a while, return to the original shape.  As mentioned earlier, if the exact delay is a half-cycle recovery time,  

Then, the wave packet contains two secondary wood of the initial wave packet, and the phase difference is half of the vibration period.  Theory suggests that for this particular  

When two wave packets propagating opposite each other meet in space, a significant interference phenomenon will be formed.  In addition, the detailed  

A detailed theoretical analysis shows that the interference fringes formed have an inverted relationship for the two adjacent encounters.  

In the experiment, the excited state wave packet dynamics of 12 vapor can be measured by means of the pump - one detection technique  

Experimental observations of the above phenomena.  Using the wide band of excitation light, excite 12 molecules to the mean vibrational quantum number,  

The vibrational coherent superposition state near 14 forms a certain domain wave packet.  For the B state of 12 molecules, the spectrum is divided  

Clearly, for the coherent superposition state of 1i=14, tcZ.3ps, Trevz37Ps have been known, and Trev>>Tc satisfies the excitation  

Conditions for generating a number of anharmonic oscillations.  Therefore, in the vicinity of Trev/4~-9.3Ps, half of the system is expected to occur  

Periodic recovery phenomenon (Figure 1.22).  Two wave packets whose phases differ by half a vibration period will oscillate in a potential well.  

The theory predicts that the nodes formed by wave packet interference will be distributed in the interval of 3.1' 3.4 cores, corresponding to two  

The interval where wave packets meet.  More importantly, the wave-packet interference fringes formed between the two adjacent regions  

A phase reversal process occurs in the interference event of, i.e., the original maximum becomes the minimum, and the minimum becomes the maximum.  

At time To, the two train wave packets are localized at the potential energy plane's point of return and propagate towards each other.  At time To 10 Tc/4, the two train packets are heavy in space  

After coherence, a steady-state wave is formed, lasting until the two waves separate.  At time To+Tc/2, the two train packets switch positions at the point of return  

And then they spread to each other again.  At time To+3Tc/4, the two train wave packets are coherent again, and the phase of the interference wave differs by two compared with the previous one (see imaginary vertical)  

Line)  

'Pumping a detection technique of up to 7gs can be used for the observation of the above interference nodes. The difficulty is how to get high enough  

Temporal and spatial resolution.  

FIG. 1.23 shows the mapping of vibration wave packet motion into measurable signals by detecting the laser in some particular  

The so-called "transient Franck-Condon point" excites the wave packet resonatively to higher excited states.  The wavelength  

The detection light induction in the range 382-391nm will produce a transition from B state to E state and record the excitation emitted by the E state  

Variation of photoinduced fluorescence intensity with pump - detection delay.  By changing the wavelength of the detection light, the selection is in different nuclei  

Spacing for excitation.  Each probe intercepts the section of the corresponding wave packet function, so that the wave packet can be detected at  

The intersection of space and space.  The detected light is a Gaussian pulse 



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