Self-Suppression of the Giant Coherent Anti-Stokes Raman Scattering Background for Detection of Buried Interfaces with Submonolayer Sensitivity.

Despite its success in many fields, the implementation of coherent anti-Stokes Raman spectroscopy (CARS) in tackling the problems at interfaces was hindered by the enormous resonant and nonresonant background from the bulk. In this work, we have developed a novel CARS scheme that can probe a buried interface via ≥105-fold suppression of the nonresonant and resonant bulk contribution. The method utilizes self-destructive interference between the forward and backward CARS generated in the bulk near the Brewster angle. As a result, we can resolve the vibrational spectrum of submonolayer interfacial polar/apolar species immersed in the surrounding medium with huge CARS responses. We expect that our approach opens up the opportunity to interrogate the interfaces involving apolar molecules and benefits other nonlinear optical spectroscopic techniques, e.g., sum-frequency spectroscopy and four-wave mixing spectroscopy in general, to promote the signal-to-background noise ratio.

Nucleation and dissociation of methane clathrate embryo at the gas-water interface.

Among natural energy resources, methane clathrate has attracted tremendous attention because of its strong relevance to current energy and environment issues. Yet little is known about how the clathrate starts to nucleate and disintegrate at the molecular level, because such microscopic processes are difficult to probe experimentally. Using surface-specific sum-frequency vibrational spectroscopy, we have studied in situ the nucleation and disintegration of methane clathrate embryos at the methane-gas-water interface under high pressure and different temperatures. Before appearance of macroscopic methane clathrate, the interfacial structure undergoes 3 stages as temperature varies, namely, dissolution of methane molecules into water interface, formation of cage-like methane-water complexes, and appearance of microscopic methane clathrate, while the bulk water structure remains unchanged. We find spectral features associated with methane-water complexes emerging in the induction time. The complexes are present over a wide temperature window and act as nuclei for clathrate growth. Their existence in the melt of clathrates explains why melted clathrates can be more readily recrystallized at higher temperature, the so-called "memory effect." Our findings here on the nucleation mechanism of clathrates could provide guidance for rational control of formation and disintegration of clathrates.

Enhancement of femtosecond surface nonlinear optical signals with spatiotemporal focusing.

Nonlinear optical reflection spectroscopies for surface studies often suffer from their relatively weak signals. To enhance the signal strength, femtosecond input pulses are generally preferred, but their intensity is usually limited by supercontinuum (SC) generation in a bulk medium. We show here that the problem can be largely resolved by means of spatiotemporal focusing (STF) of the input that effectively suppresses the SC, permits much higher input intensity on a medium, and greatly enhances the nonlinear output signal. We use second-harmonic generation (SHG) on oxide as examples and demonstrate that, with STF, the input intensity for the onset of the SC can be increased by ∼30 times in comparison with conventional focusing, and the reflected SHG can be increased by about three orders of magnitude.

Programmable graphene nanobubbles with three-fold symmetric pseudo-magnetic fields.

Graphene nanobubbles (GNBs) have attracted much attention due to the ability to generate large pseudo-magnetic fields unattainable by ordinary laboratory magnets. However, GNBs are always randomly produced by the reported protocols, therefore, their size and location are difficult to manipulate, which restricts their potential applications. Here, using the functional atomic force microscopy (AFM), we demonstrate the ability to form programmable GNBs. The precision of AFM facilitates the location definition of GNBs, and their size and shape are tuned by the stimulus bias of AFM tip. With tuning the tip voltage, the bubble contour can gradually transit from parabolic to Gaussian profile. Moreover, the unique three-fold symmetric pseudo-magnetic field pattern with monotonous regularity, which is only theoretically predicted previously, is directly observed in the GNB with an approximately parabolic profile. Our study may provide an opportunity to study high magnetic field regimes with the designed periodicity in two dimensional materials.

Phase reference in phase-sensitive sum-frequency vibrational spectroscopy.

Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) has been established as a powerful technique for surface characterization, but for it to generate a reliable spectrum, accurate phase measurement with a well-defined phase reference is most important. Incorrect phase measurement can lead to significant distortion of a spectrum, as recently seen in the case for the air/water interface. In this work, we show theoretically and experimentally that a transparent, highly nonlinear crystal, such as quartz and barium borate, can be a good phase reference if the surface is clean and unstrained and the crystal is properly oriented to yield a strong SF output. In such cases, the reflected SF signal is dominated by the bulk electric dipole contribution and its phase is either +90° or -90°. On the other hand, materials with inversion symmetry, such as water, fused quartz, and CaF2 are not good phase references due to the quadrupole contribution and phase dispersion at the interface. Using a proper phase reference in PS-SFVS, we have found the most reliable OH stretching spectrum for the air/water interface. The positive band at low frequencies in the imaginary component of the spectrum, which has garnered much interest and been interpreted by many to be due to strongly hydrogen-bonded water species, is no longer present. A weak positive feature however still exists. Its magnitude approximately equals to that of air/D2O away from resonances, suggesting that this positive feature is unrelated to surface resonance of water.

Effects of job rotation and role stress among nurses on job satisfaction and organizational commitment.

BACKGROUND: The motivation for this study was to investigate how role stress among nurses could affect their job satisfaction and organizational commitment, and whether the job rotation system might encourage nurses to understand, relate to and share the vision of the organization, consequently increasing their job satisfaction and stimulating them to willingly remain in their jobs and commit themselves to the organization. Despite the fact that there have been plenty of studies on job satisfaction, none was specifically addressed to integrate the relational model of job rotation, role stress, job satisfaction, and organizational commitment among nurses. METHODS: With top managerial hospital administration's consent, questionnaires were only distributed to those nurses who had had job rotation experience. 650 copies of the questionnaire in two large and influential hospitals in southern Taiwan were distributed, among which 532 valid copies were retrieved with a response rate of 81.8%. Finally, the SPSS 11.0 and LISREL 8.54 (Linear Structural Relationship Model) statistical software packages were used for data analysis and processing. RESULTS: According to the nurses' views, the findings are as follows: (1) job rotation among nurses could have an effect on their job satisfaction; (2) job rotation could have an effect on organizational commitment; (3) job satisfaction could have a positive effect on organizational commitment; (4) role stress among nurses could have a negative effect on their job satisfaction; and (5) role stress could have a negative effect on their organizational commitment. CONCLUSION: As a practical and excellent strategy for manpower utilization, a hospital could promote the benefits of job rotation to both individuals and the hospital while implementing job rotation periodically and fairly. And when a medical organization attempts to enhance nurses' commitment to the organization, the findings suggest that reduction of role ambiguity in role stress has the best effect on enhancing nurses' organizational commitment. The ultimate goal is to increase nurses' job satisfaction and encourage them to stay in their career. This would avoid the vicious circle of high turnover, which is wasteful of the organization's valuable human resources.