Rydberg state dynamics and fragmentation mechanism of N,N,N',N'-tetramethylmethylenediamine.

The non-adiabatic relaxation processes and the fragmentation dynamics of Rydberg-excited N,N,N',N'-tetramethylmethylenediamine (TMMDA) are investigated using femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. Excitation at 208 nm populates TMMDA in a charge-localized 3p state. Rapid internal conversion (IC) to 3s produces two charge-delocalized conformers with independent time constants and distinct population ratios. As the system explores the 3s potential surface, the structural evolution continues on a 1.55 ps timescale, followed by a slower (12.1 ps) relaxation to the ground state. A thorough comparison of the time-dependent mass and photoelectron spectra suggests that ionization out of the 3p state ends up with the parent ion, the vibrational energy of which is insufficient for the bond cleavage. On the contrary, by virtue of the additional energy acquired by IC from 3p, the internal energy deposited in 3s is available to break the C-N bond, leading to the fragment ion. The fragmentation is found to occur on the ion surface instead of the Rydberg surface.

Two-watt mid-infrared laser emission in robust fluorozirconate fibers.

To the best of our knowledge, we report here the first demonstration of 2.9 µm laser emission from in-house fabricated Ho3+/Pr3+ co-doped ZBYA glass fiber. The fiber was fabricated based on the ZBYA glass with compositions of ZrF4-BaF2-YF3-AlF3-PbF2-HoF3-PrF3. Under the pump of a 1150 nm Raman fiber laser, the maximum unsaturated output power of 2.16 W was obtained in a 15 cm long gain fiber with a slope efficiency of 24%. The influence of rare-earth doping concentration on laser performance was also investigated. The result indicates that ZBYA glass fibers have potential for using as a fluorozirconate glass gain fiber for mid-infrared fiber lasers.

Intersystem Crossing of 2-Methlypyrazine Studied by Femtosecond Photoelectron Imaging.

2-methylpyrazine was excited to the high vibrational dynamics of the S1 state with 260 nm femtosecond laser light, and the evolution of the excited state was probed with 400 nm light. Because it was unstable, the S1 state decayed via intersystem crossing to the triplet state T1, and it may have decayed to the ground state S0 via internal conversion. S1-to-T1 intersystem crossing was observed by combining time-resolved mass spectrometry and time-resolved photoelectron spectroscopy. The crossover time scale was 23 ps. Rydberg states were identified, and the photoelectron spectral and angular distributions indicated accidental resonances of the S1 and T1 states with the 3s and 3p Rydberg states, respectively, during ionization.

Strain-, curvature- and twist-independent temperature sensor based on a small air core hollow core fiber structure.

Cross-sensitivity (crosstalk) to multiple parameters is a serious but common issue for most sensors and can significantly decrease the usefulness and detection accuracy of sensors. In this work, a high sensitivity temperature sensor based on a small air core (10 µm) hollow core fiber (SACHCF) structure is proposed. Co-excitation of both anti-resonant reflecting optical waveguide (ARROW) and Mach-Zehnder interferometer (MZI) guiding mechanisms in transmission are demonstrated. It is found that the strain sensitivity of the proposed SACHCF structure is decreased over one order of magnitude when a double phase condition (destructive condition of MZI and resonant condition of ARROW) is satisfied. In addition, due to its compact size and a symmetrical configuration, the SACHCF structure shows ultra-low sensitivity to curvature and twist. Experimentally, a high temperature sensitivity of 31.6 pm/°C, an ultra-low strain sensitivity of -0.01pm/µÎµ, a curvature sensitivity of 18.25 pm/m-1, and a twist sensitivity of -22.55 pm/(rad/m) were demonstrated. The corresponding temperature cross sensitivities to strain, curvature and twist are calculated to be -0.00032 °C/µÎµ, 0.58 °C/m-1 and 0.71 °C/(rad/m), respectively. The above cross sensitivities are one to two orders of magnitude lower than that of previously reported optical fiber temperature sensors. The proposed sensor shows a great potential to be used as a temperature sensor in practical applications where influence of multiple environmental parameters cannot be eliminated.

Negative Curvature Hollow Core Fiber Based All-Fiber Interferometer and Its Sensing Applications to Temperature and Strain.

Negative curvature hollow core fiber (NCHCF) is a promising candidate for sensing applications; however, research on NCHCF based fiber sensors starts only in the recent two years. In this work, an all-fiber interferometer based on an NCHCF structure is proposed for the first time. The interferometer was fabricated by simple fusion splicing of a short section of an NCHCF between two singlemode fibers (SMFs). Both simulation and experimental results show that multiple modes and modal interferences are excited within the NCHCF structure. Periodic transmission dips with high spectral extinction ratio (up to 30 dB) and wide free spectral range (FSR) are produced, which is mainly introduced by the modes coupling between HE11 and HE12. A small portion of light guiding by means of Anti-resonant reflecting optical waveguide (ARROW) mechanism is also observed. The transmission dips, resulting from multimode interferences (MMI) and ARROW effect have a big difference in sensitivities to strain and temperature, thus making it possible to monitor these two parameters with a single sensor head by using a characteristic matrix approach. In addition, the proposed sensor structure is experimentally proven to have a good reproducibility.

Strain independent twist sensor based on uneven platinum coated hollow core fiber structure.

Optical fiber based twist sensors usually suffer from high cross sensitivity to strain. Here we report a strain independent twist sensor based on an uneven platinum coated hollow core fiber (HCF) structure. The sensor is fabricated by splicing a section of ~4.5-mm long HCF between two standard single mode fibers, followed by a sputter-coating of a very thin layer of platinum on both sides of the HCF surface. Experimental results demonstrate that twist angles can be measured by monitoring the strength change of transmission spectral dip. The sensor's cross sensitivity to strain is investigated before and after coating with platinum. It is found that by coating a platinum layer of ~9 nm on the HCF surface, the sensor's cross sensitivity to strain is significantly decreased with over two orders of magnitude less than that of the uncoated sensor sample. The lowest strain sensitivity of ~2.32×10-5 dB/𝜇𝛆 has been experimentally achieved, which is to the best of our knowledge, the lowest cross sensitivity to strain reported to date for optical fiber sensors based on intensity modulation. In addition, the proposed sensor is capable of simultaneous measurement of strain and twist angle by monitoring the wavelength shift and dip strength variation of a single spectral dip. In the experiment, strain and twist angle sensitivities of 0.61 pm/𝜇𝛆 and 0.10 dB/° have been achieved. Moreover, the proposed sensor offers advantages of ease of fabrication, miniature size, and a good repeatability of measurement.

Sub-micrometer resolution liquid level sensor based on a hollow core fiber structure.

Liquid level measurement in lab on a chip (LOC) devices is a challenging task due to the demand for a sensor with ultra-high resolution but miniature in nature. In this Letter, we report a simple, compact in size, yet highly sensitive liquid level sensor based on a hollow core fiber (HCF) structure. The sensor is fabricated by fusion splicing a short section of HCF between two singlemode fibers (SMFs). Sensor samples with different lengths of HCF have been studied; it is found that the sensor with a HCF length of ∼4.73 mm shows the best sensitivity of ∼0.014 dB/µm, corresponding to a liquid level resolution of ∼0.7 µm, which is over five times higher than that of the previous reported fiber optic sensors to date. In addition, experimental results have demonstrated that the proposed sensor shows good repeatability of measurement and a very low cross sensitivity to changes in the refractive index of the surrounding medium.

Vibrational coherence in the composition-selected wavepacket of photoexcited pyrimidine.

Coherent wavepacket motion in photoexcited pyrimidine has been initiated and visualized in real time using femtosecond time-resolved ion-yield spectroscopy. A coherent superposition of at least four low-frequency Frank-Condon (FC) active modes is created in the first excited electronic state (S1), leading to a vibrational wavepacket. Its composition is manipulated experimentally by tuning the excitation wavelength in the range 309-313 nm to populate the selected vibrational levels. Interference among these vibrational levels is directly characterized by a clear quantum beat superimposed on a single-exponential decay. Fourier transform analysis of the wavelength-dependent transients shows modulation at different frequencies, providing a direct signature of multi-mode vibrational coherence resulting from the coherent excitation process. The sensitivity of the parent-ion transient to the vibrational wavepacket dynamics probably arises because different modes are connected by variable FC factors to the 3s and 3p Rydberg states.

Femtosecond real-time probing of the excited-state intramolecular proton transfer reaction in methyl salicylate.

The excited-state intramolecular proton transfer (ESIPT) process and subsequent electronic relaxation dynamics in methyl salicylate have been investigated using femtosecond time-resolved ion yield spectroscopy combined with time-resolved photoelectron imaging. Excitation with a tunable pump pulse populates the keto tautomer in the first excited electronic state S1(ππ*). As a hydrogen atom transfers from the phenolic group to the carbonyl group within 100 fs, the molecular geometry changes gradually, leading to a variation in the electronic photoionization channel. By virtue of the accidental resonance with some intermediate Rydberg states, the time-dependent photoelectron spectra provide a direct mapping of the ESIPT reaction from the initially populated keto tautomer to the proton-transferred enol tautomer. Subsequently, the population around the enol configuration undergoes intramolecular vibrational redistribution on a subpicosecond time scale, followed by internal conversion to the ground state with a wavelength-dependent lifetime in the picosecond range. Furthermore, the excitation energies of several Rydberg states in methyl salicylate are determined experimentally.

Unraveling the electronic relaxation dynamics in photoexcited 2,4-difluoroaniline via femtosecond time-resolved photoelectron imaging.

Time-resolved photoelectron imaging is employed to investigate the relaxation dynamics of the lowest two excited electronic states S1(ππ*) and S2(π3s/πσ*) in 2,4-difluoroaniline (24DFA). As the S1(ππ*) state is populated directly following 289 nm excitation, the population undergoes ultrafast intramolecular vibrational redistribution on a 540 fs time scale, followed by efficient intersystem crossing from S1(ππ*) to the triplet state within 379 ps, and the subsequent slower deactivation process of the triplet state. For excitation to the S2(π3s/πσ*) state at 238 nm, the population probably bifurcates into two decay channels. The dominant channel with 84 fs involves ultrafast internal conversion to the S1(ππ*) state, from which it relaxes to the electronic ground state on a 116 ps time scale. The other appears to involve motion along the S2(π3s/πσ*) potential energy surface. Our data also determine experimentally the electronic energies of S2(π3s/πσ*), S3(ππ*), and several Rydberg states in 24DFA.

Real-time visualization of the vibrational wavepacket dynamics in electronically excited pyrimidine via femtosecond time-resolved photoelectron imaging.

The vibrational wavepacket dynamics at the very early stages of the S1-T1 intersystem crossing in photoexcited pyrimidine is visualized in real time by femtosecond time-resolved photoelectron imaging and time-resolved mass spectroscopy. A coherent superposition of the vibrational states is prepared by the femtosecond pump pulse at 315.3 nm, resulting in a vibrational wavepacket. The composition of the prepared wavepacket is directly identified by a sustained quantum beat superimposed on the parent-ion transient, possessing a frequency in accord with the energy separation between the 6a1 and 6b2 states. The dephasing time of the vibrational wavepacket is determined to be 82 ps. More importantly, the variable Franck-Condon factors between the wavepacket components and the dispersed cation vibrational levels are experimentally illustrated to identify the dark state and follow the energy-flow dynamics on the femtosecond time scale. The time-dependent intensities of the photoelectron peaks originated from the 6a1 vibrational state exhibit a clear quantum beating pattern with similar periodicity but a phase shift of π rad with respect to those from the 6b2 state, offering an unambiguous picture of the restricted intramolecular vibrational energy redistribution dynamics in the 6a1/6b2 Fermi resonance.

Structural dynamics upon photoinduced charge transfer in N,N,N',N'-tetramethylmethylenediamine.

The ultrafast structural motion linked to the charge transfer process in Rydberg excited N,N,N',N'-tetramethylmethylenediamine (TMMDA) has been monitored in real time using femtosecond time-resolved photoelectron imaging coupled with quantum chemical calculations. Optical excitation to the 3 s Rydberg state initially populates the charge on one of the two amine groups, resulting in a charge-localized structure in the Franck-Condon (FC) region. As the wavepacket evolves on the 3 s potential surface, the molecular geometry changes with time, leading to the corresponding variation in the charge distribution. The ensuing structural evolution yields two distinct conformers GG+ and TT+ (see text for nomenclature), both with the charge delocalized between the two nitrogen atoms. By virtue of the sensitivity of the Rydberg electron binding energy (BE) on the nuclear geometry, the time-dependent BE spectrum offers an intuitive mapping of the charge transfer reaction that leads from the initially prepared charge-localized GG-FC structure to the fully charge-delocalized GG+ and TT+ structures. Complementary computations provide evidence that through-space interaction is responsible for the charge delocalization in the GG+ and TT+ structures.

Imaging Reversible and Irreversible Structural Evolution in Photoexcited 2,4-Difluoroaniline.

Here, we demonstrate the capability of femtosecond time-resolved photoelectron imaging to visualize the reversible and irreversible structural evolution in electronically excited 2,4-difluoroaniline. As a coherent superposition of out-of-plane vibrational motions is created following 299.8 nm excitation, the molecular geometry alters periodically, thus modulating the photoionization channel. The reversible nuclear motion between distinct geometries is directly observed as the reverse beats in the time-dependent photoelectron spectra. Moreover, with the molecule highly vibrationally excited at 289.0 nm, the exponential decrease/increase changes in photoelectron signal provide a physically intuitive and complete picture of the irreversible geometry rearrangement away from a nonplanar geometry in the vertical Franck-Condon region toward the planar minimum.

Femtosecond time-resolved observation of butterfly vibration in electronically excited o-fluorophenol.

The butterfly vibration during the hydrogen tunneling process in electronically excited o-fluorophenol has been visualized in real time by femtosecond time-resolved ion yield spectroscopy coupled with time-resolved photoelectron imaging technique. A coherent superposition of out-of-plane C-F butterfly motions is prepared in the first excited electronic state (S1). As the C-F bond vibrates with respect to the aromatic ring, the nuclear geometry varies periodically, leading to the corresponding variation in the photoionization channel. By virtue of the more favorable ionization probability from the nonplanar minimum via resonance with the Rydberg states, the evolution of the vibrational wave packet is manifested as a superimposed beat in the parent-ion transient. Moreover, time-resolved photoelectron spectra offer a direct mapping of the oscillating butterfly vibration between the planar geometry and nonplanar minimum. The beats for the photoelectron peaks originating from the planar geometry are out of phase with those from the nonplanar minimum. Our results provide a physically intuitive and complete picture of the oscillatory flow of energy responsible for the coherent vibrational motion on the excited state surface.