Evidence of a Hardening in the Cosmic Ray Proton Spectrum at around 166 TeV Observed by the GRAPES-3 Experiment.

We present the measurement of the cosmic ray proton spectrum from 50 TeV to 1.3 PeV using 7.81×10^{6} extensive air shower events recorded by the ground-based GRAPES-3 experiment between 1 January 2014 and 26 October 2015 with a live time of 460 day. Our measurements provide an overlap with direct observations by satellite and balloon-based experiments. The electromagnetic and muon components in the shower were measured by a dense array of plastic scintillator detectors and a tracking muon telescope, respectively. The relative composition of the proton primary from the air shower data containing all primary particles was extracted using the multiplicity distribution of muons which is a sensitive observable for mass composition. The observed proton spectrum suggests a spectral hardening at ∼166 TeV and disfavors a single power law description of the spectrum up to the Knee energy (∼3 PeV).

EXtraction or PREServation? EXPRESS survey of patients' preference for toothache in public health facilities of Eastern India.

OBJECTIVE: To investigate patients' preference for extraction or preservation for toothache and hypothetical anterior tooth pain along with the specific reason for their choice. BASIC RESEARCH DESIGN: Cross-sectional analytical semi-structured interview study. PARTICIPANTS: A sample of 703 adult dental outpatients visiting secondary and tertiary government health centres with toothache due to dental caries in Eastern India. MAIN OUTCOME MEASURES: Patients preferring restorative or extraction services for toothache, specific reason, and socio-demographic background factors for anterior and posterior teeth. RESULTS: Half (50.1%) choose preservation for present toothache and 79.9% for hypothetical front tooth pain. Immediate relief from toothache for extraction and the motive to preserve natural teeth for preservation were the main reasons expressed. In logistic regression, participants preferring extraction were more likely to be aged 25-34 years (OR = 1.94), 55+ years (OR=33.32), have primary and below education level (OR=1.99), have had a previous extraction (OR=1.99) and be unaware of preservation options (OR=2.34). For assumed anterior tooth pain, those between 25-34 years (OR=0.39) were more likely to choose preservation. Participants with primary and below education levels (OR=1.99) and unaware of preservation options (OR=1.95) chose extraction of the front tooth irrespective of their choice of treatment for the present toothache. CONCLUSION: Notable differences between the choices to preserve or extract a posterior tooth were not found. There was greater preference towards preserving anterior teeth. Future research should identify additional barriers to the preference and utilization of restorative services.

Electronic nonadiabatic effects in the state-to-state dynamics of the H + H2 → H2 + H exchange reaction with a vibrationally excited reagent.

Out of the many major breakthroughs that the hydrogen-exchange reaction has led to, electronic nonadiabatic effects that are mainly due to the geometric phase has intrigued many. In this work we investigate such effects in the state-to-state dynamics of the H + H2 (v = 3, 4, j = 0) → H2 (v', j') + H reaction with a vibrationally excited reagent at energies corresponding to thermal conditions. The dynamical calculations are performed by a time-dependent quantum mechanical method both on the lower adiabatic potential energy surface (PES) and also using a two-states coupled diabatic theoretical model to explicitly include all the nonadiabatic couplings present in the 1E' ground electronic manifold of the H3 system. The nonadiabatic couplings are considered here up to the quadratic term; however, the effect of the latter on the reaction dynamics is found to be very small. Adiabatic population analysis showed a minimal participation of the upper adiabatic surface even for the vibrationally excited reagent. A strong nonadiabatic effect appears in the state-to-state reaction probabilities and differential cross sections (DCSs). This effect is manifested as "out-of-phase" oscillations in the DCSs between the results of the uncoupled and coupled surface situations. The oscillations persist as a function of both scattering angle and collision energy in both the backward and forward scattering regions. The origins of these oscillations are examined in detail. The oscillations that appear in the forward direction are found to be different from those due to glory scattering, where the latter showed a negligibly small nonadiabatic effect. The nonadiabatic effects are reduced to a large extent when summed over all product quantum states, in addition to the cancellation due to integration over the scattering angle and partial wave summation.

Combined Quantum Mechanical and Quasi-Classical State-to-State Dynamical Study on the Isotopic Effect in H/D + LiH+/LiD+ → H2/HD/D2 + Li+ Reactions.

Coriolis-coupled quantum mechanical (QM-CC) and quasi-classical trajectory (QCT) calculations are carried out to investigate the dynamics of the H(D) + LiH+(v = 0, j = 0) → H2(HD) (v', j') + Li+ reactions on the ground electronic state potential energy surface reported by Martinazzo et al. (Martinazzo et al., J. Chem. Phys. 2003, 119, 11241). The QM-CC and QCT results at the initial state-selected and state-to-state levels are used to investigate the validity and accuracy of the QCT method for these exoergic barrierless reactions. Furthermore, the QCT method is used to understand the isotopic effects on reaction observables like total and state-to-state integral cross section, differential cross section, product energy disposal, and rate constants of H(D) + LiH+(v = 0, j = 0) → H2(HD) (v', j') + Li+ and H(D) + LiD+(v = 0, j = 0) → HD(D2) (v', j') + Li+ reactions. Attempts are also made to understand the impact of the isotopic substitution on the reaction mechanism. It is observed that QM-CC and QCT results closely follow each other at the initial state-selected and state-to-state levels. Noticeable kinematic effects of reagents on the reactivity and mechanism of the reactions are also observed.

Asymmetric comb waveguide for strong interactions between atoms and light.

Coupling quantum emitters and nanostructures, in particular cold atoms and optical waveguides, has recently raised a large interest due to unprecedented possibilities of engineering light-matter interactions. In this work, we propose a new type of periodic dielectric waveguide that provides strong interactions between atoms and guided photons with an unusual dispersion. We design an asymmetric comb waveguide that supports a slow mode with a quartic (instead of quadratic) dispersion and an electric field that extends far into the air cladding for an optimal interaction with atoms. We compute the optical trapping potential formed with two guided modes at frequencies detuned from the atomic transition. We show that cold Rubidium atoms can be trapped as close as 100 nm from the structure in a 1.3-mK-deep potential well. For atoms trapped at this position, the emission into guided photons is largely favored, with a beta factor as high as 0.88 and a radiative decay rate into the slow mode 10 times larger than the free-space decay rate. These figures of merit are obtained at a moderately low group velocity of c/50.

The Jahn-Teller and pseudo-Jahn-Teller effects in the propyne radical cation.

The Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in the X̃2E, Ã2E and B̃2A1 electronic states of the propyne radical cation are investigated with the aid of ab initio quantum chemistry calculations and first principles quantum dynamics simulations. For the latter, both time-independent and time-dependent quantum mechanical methods are employed. Standard vibronic coupling theory is used to construct a symmetry consistent vibronic Hamiltonian in a diabatic electronic basis. Taylor series expansion of the elements of the diabatic electronic Hamiltonian is carried out and the parameters that appear in the expansion are derived from the ab initio calculated adiabatic electronic energies. It is found that the JT effect is weak in the X̃2E state as compared to that in the Ã2E state. Because of large energy separation, the PJT coupling among the JT-split components of the X̃2E state with the neighboring states is also very weak. However, the PJT coupling of the B̃2A1 state with the JT split components of the Ã2E state has some impact on the dynamics in the coupled Ã2E-B̃2A1 electronic states. The vibronic spectrum of each of these states is calculated and compared with the experimental results. The nonradiative internal conversion dynamics of excited cationic states is examined. Interesting comparison is made with the JT and PJT coupling effects in the nuclear dynamics of the X̃2E-Ã2E-B̃2B2 electronic states of the isomeric allene radical cation.

Multi-state and Multi-mode Vibronic Coupling Effects in the Photoionization Spectroscopy of Acetaldehyde.

Multi-state and multi-mode vibronic dynamics in the seven energetically low-lying (X~2A', A~2A″, B~2A', C~2A', D~2A″, E~2A', and F~2A') electronic states of the acetaldehyde radical cation is theoretically studied in this article. Adiabatic energies of these electronic states are calculated by ab initio quantum chemistry methods. A vibronic coupling model of seven electronic states is constructed in a diabatic electronic basis to carry out the first-principles nuclear dynamics study. The vibronic spectrum is calculated and compared with the experimental findings reported in the literature. The progressions of vibrational modes found in the spectrum are assigned. The findings reveal that the X~2A' and F~2A' electronic states are energetically well-separated from the other electronic states and the remaining states (A~2A″ to E~2A') are energetically very close or even quasi-degenerate at the equilibrium geometry of the reference electronic ground state of acetaldehyde. The energetic proximity of A~2A″ to E~2A' electronic states results in multiple multi-state conical intersections. The impact of electronic nonadiabatic interactions due to conical intersections on the vibronic structure of the photoionization band and nonradiative internal conversion dynamics is discussed.

Optimal control of photodissociation of phenol using genetic algorithm.

Photodissociation dynamics of the OH bond of phenol is studied with an optimally shaped laser pulse. The theoretical model consists of three electronic states (the ground electronic state, ππ* state, and πσ* state) in two nuclear coordinates (the OH stretching coordinate as a reaction coordinate, r, and the CCOH dihedral angle as a coupling coordinate, θ). The optimal UV laser pulse is designed using the genetic algorithm, which optimizes the total dissociative flux of the wave packet. The latter is calculated in the adiabatic asymptotes of the S0 and S1 electronic states of phenol. The initial state corresponds to the vibrational levels of the electronic ground state and is defined as |nr, nθ⟩, where nr and nθ represent the number of nodes along r and θ, respectively. The optimal UV field excites the system to the optically dark πσ* state predominantly over the optically bright ππ* state with the intensity borrowing effect for the |0, 0⟩ and |0, 1⟩ initial states. For the |0, 0⟩ initial condition, the photodissociation to the S1 asymptotic channel is favored slightly over the S0 asymptotic channel. Addition of one quantum of energy along the coupling coordinate increases the dissociation probability in the S1 channel. This is because the wave packet spreads along the coupling coordinate on the πσ* state and follows the adiabatic path. Hence, the S1 asymptotic channel gets more (∼11%) dissociative flux as compared to the S0 asymptotic channel for the |0, 1⟩ initial condition. The |1, 0⟩ and |1, 1⟩ states are initially excited to both the ππ* and πσ* states in the presence of the optimal UV pulse. For these initial conditions, the S1 channel gets more dissociative flux as compared to the S0 channel. This is because the high energy components of the wave packet readily reach the S1 channel. The central frequency of the optimal UV pulse for the |0, 0⟩ and |0, 1⟩ initial states has a higher value as compared to the |1, 0⟩ and |1, 1⟩ initial states. This is explained with the help of an excitation mechanism of a given initial state in relation to its energy.

Elucidation of vibronic structure and dynamics of first eight excited electronic states of pentafluorobenzene.

Vibronic coupling in the first eight electronic excited states of Pentafluorobenzene (PFBz) is investigated in this article. In particular, the vibronic coupling between the optically bright ππ* and optically dark πσ* states of PFBz is considered. A model 8 × 8 diabatic Hamiltonian is constructed in terms of normal coordinate of vibrational modes using the standard vibronic coupling theory and symmetry selection rule. The Hamiltonian parameters are estimated with the aid of extensive ab initio quantum chemistry calculations. The topography of the first eight electronic excited states of PFBz is examined at length, and multiple multi-state conical intersections are established. The nuclear dynamics calculations on the coupled electronic surfaces are carried out from first principles by the wave packet propagation method. Theoretical results are found to be in good accord with the available experimental optical absorption spectrum of PFBz.

Targeted (PCR-based) screening of antibiotic resistance genes' prevalence in the gut microbiota of tribal people of Nabarangpur, Odisha, India.

Antibiotic resistance is a major public health concern worldwide. The gut microbiota harbours multiple antibiotic resistant genes (ARGs) that contribute to the existing and future microbial population in a community or ecosystem. This study aimed to investigate the prevalence of 35 antibiotic resistance genes (ARGs) in the gut microbiota of the tribal people of Nabarangpur, Odisha, India. A total of 83 faecal samples were collected from three different tribes (Bhatra, Gond, and Paraja). Total faecal DNA was extracted, and the simplex polymerase chain reaction was performed to detect selected ARGs. Further analysis was done to estimate the incidence of these ARGs across these tribes based on alcohol consumption habits. We identified a higher prevalence of tetracycline resistance genes (tetW, tetQ and tetM) in the gut microbiota among three populations. Furthermore, a significant (P = 0·024) difference in ARG prevalence against vancomycin in individuals with and without alcohol consumption habits was noticed. The overall distribution of ARGs among the three major tribes of this location was found to be very similar. Together, irrespective of the tribes, the people of this location have gut microbiota harbouring different kinds of ARGs and tetracycline-resistant genes are the most commonly found ARGs.

Femtosecond Transfer and Manipulation of Persistent Hot-Trion Coherence in a Single CdSe/ZnSe Quantum Dot.

Ultrafast transmission changes around the fundamental trion resonance are studied after exciting a p-shell exciton in a negatively charged II-VI quantum dot. The biexcitonic induced absorption reveals quantum beats between hot-trion states at 133 GHz. While interband dephasing is dominated by relaxation of the P-shell hole within 390 fs, trionic coherence remains stored in the spin system for 85 ps due to Pauli blocking of the triplet electron. The complex spectrotemporal evolution of transmission is explained analytically by solving the Maxwell-Liouville equations. Pump and probe polarizations provide full control over amplitude and phase of the quantum beats.

Quantum interference in the mechanism of H + LiH+ → H2 + Li+ reaction dynamics.

In this work, the detailed reaction mechanism of the astrochemically relevant exoergic and barrierless H + LiH+ → H2 + Li+ reaction is investigated by both time-dependent wave packet and quasi-classical trajectory (QCT) methods on the ab initio electronic ground state potential energy surface reported by Martinazzo et al. [Martinazzo et al., J. Chem. Phys., 2003, 119, 11241]. The interference terms due to the coherence between the partial waves are quantified. When plotted along the scattering angle they reveal interference of constructive or destructive nature. Significant interference was found in the differential cross-section (DCS) which is a symbolic of the non-statistical nature of the reaction. This is further complemented by calculating the lifetime of the collision complex by the QCT method. It is found that the reaction follows a direct stripping mechanism at higher collision energies and yields forward scattered products from collisions involving high total angular momentum. At low collision energies, the reaction follows a mixed direct/indirect mechanism but with a dominant indirect contribution. The product state-resolved DCSs reveal that two opposite mechanisms co-exist, both at low and high collision energies. The microscopic scattering mechanism of the reaction is found to be unaffected by the ro-vibrational excitation of the reagent diatom.

Theoretical Study of the Energy Disposal Mechanism and the State-Resolved Quantum Dynamics of the H + LiH+ → H2 + Li+ Reaction.

Despite several studies in the literature, the detailed quantum state-to-state level mechanism of the astrophysically important exoergic barrierless H + LiH+ → H2 + Li+ reaction is yet to be understood. In this work, we have investigated the energy disposal mechanism of the reaction in terms of integral reaction cross section, product internal state distributions, differential cross section, and rate constant. Fully converged and Coriolis coupled quantum mechanical calculations based on a time-dependent wave packet method have been performed at the state-to-state level on the ab initio electronic ground state potential energy surface (PES) constructed by Martinazzo et al. (J. Chem. Phys. 2003, 119, 11241-11248). The agreement between the present quantum mechanical and previous quasi-classical results is found even at very low relative translational energies of reagents. A non-statistical inverse Boltzmann vibrational distribution for the product is found. This is attributed to the "attractive" nature of the underlying PES, which facilitates the excess energy release mostly as product vibration (60-80%). The energy disposal in products is found to be unaffected by the rovibrational excitation of the reagent diatom due to the weak coupling between the vibrational modes of the reagent and the product. The mild effect of collision energy on the product energy disposal is ascribed to the effective coupling between the translational modes of the reagent and the product. It is found that the collisions lead to the formation of the product H2 in its rovibrationally excited levels.

Vibronic coupling in the first six electronic states of pentafluorobenzene radical cation: Radiative emission and nonradiative decay.

Nuclear dynamics in the first six vibronically coupled electronic states of pentafluorobenzene radical cation is studied with the aid of the standard vibronic coupling theory and quantum dynamical methods. A model 6 × 6 vibronic Hamiltonian is constructed in a diabatic electronic basis using symmetry selection rules and a Taylor expansion of the elements of the electronic Hamiltonian in terms of the normal coordinate of vibrational modes. Extensive ab initio quantum chemistry calculations are carried out for the adiabatic electronic energies to establish the diabatic potential energy surfaces and their coupling surfaces. Both time-independent and time-dependent quantum mechanical methods are employed to perform nuclear dynamics calculations. The vibronic spectrum of the electronic states is calculated, assigned, and compared with the available experimental results. Internal conversion dynamics of electronic states is examined to assess the impact of various couplings on the nuclear dynamics. The impact of increasing fluorination of the parent benzene radical cation on its radiative emission is examined and discussed.

Effect of Reagent Vibration and Rotation on the State-to-State Dynamics of the Hydrogen Exchange Reaction, H + H2 → H2 + H.

State-to-state dynamics of the benchmark hydrogen exchange reaction H + H2 (v = 0-4, j = 0-3) → H2 (v', j') + H is investigated with the aid of the real wave packet approach of Gray and Balint-Kurti (J. Chem. Phys. 1998, 108, 950-962) and electronic ground BKMP2 potential energy surface of Boothroyd et al. (J. Chem. Phys. 1996, 104, 7139-7152). Initial state-selected and product state-resolved reaction probabilities, integral cross section, and product diatom vibrational and rotational level populations at a few collision energies are reported to elucidate the energy disposal mechanism. State-specific thermal rate constants are also calculated and compared with the available literature results. Coriolis coupling terms of the nuclear Hamiltonian are included, and calculations are parallelized over the helicity quantum number, Ω'. Attempts are made, in particular, to study the effect of reagent vibrational and rotational excitations on the dynamical attributes. It is found that the calculations become computationally expensive with reagent vibrational and rotational excitation. Reagent vibrational excitation is found to enhance the reactivity and has significant impact on the energy disposal to the vibrational and rotational degrees of freedom of the product. The interplay of reagent translational and vibrational energy on the product vibrational distribution unfolds an important aspect of the energy disposal mechanism. The effect of reagent rotation on the state-to-state dynamics is found not to be very significant, and the weak effect turns out to be specific to v'.

Controlled intramolecular H-transfer in malonaldehyde in the electronic ground state mediated through the conical intersection of 1nπ* and 1ππ* excited electronic states.

We report photo-isomerization of malonaldehyde in its electronic ground state (S0), mediated by coupled 1nπ*(S1)-1ππ*(S2) excited electronic states, accomplished with the aid of optimally designed ultraviolet (UV)-laser pulses. In particular, control of H-transfer from a configuration predominantly located in the left well (say, reactant) to that in the right well (say, product) of the electronic ground S0 potential energy surface is achieved by a pump-dump mechanism including the nonadiabatic interactions between the excited S1 and S2 states. An interplay between the nonadiabatic coupling due to the conical intersection of the S1 and S2 states and the laser-molecule interaction is found to be imprinted in the time-dependent electronic population. The latter is also examined by employing optimal fields of varying intensities and frequencies of the UV laser pulses. For the purpose of the present study, we constructed a three-state and two-mode coupled diabatic Hamiltonian with the help of adiabatic electronic energies and transition dipole moments calculated by ab initio quantum chemistry methods. The electronic diabatic model is developed using the calculated adiabatic energies of the two excited electronic states (S1 and S2) in order to carry out the dynamics study. The optimal fields for achieving the controlled isomerization are designed within the framework of optimal control theory employing the optimization technique of a multitarget functional using the genetic algorithm. The laser-driven dynamics of the system is treated by numerically solving the time-dependent Schrödinger equation within the dipole approximation. A time-averaged yield of the target product of ∼40% is achieved in the present treatment of dynamics with optimal laser pulses.

Work-rest pattern, alertness and performance assessment among naval personnel deployed at sea: A cross sectional study.

BACKGROUND: The combat naval vessels require continuous operational preparedness for guarding the vast coastline. Sailors onboard these ships are at risk for sleep deprivation. Sailors may be vulnerable to chronic fatigue; and decreased performance, vigilance, and cognitive functioning owing to sleep debt, which may affect safety during tactical operations and while transportation of ships. In this study we assessed the work-rest pattern and level of alertness during sailing and non sailing days. METHODS: Thirty-two male participants maintained a 24-h sleep and activity log for 5 days. Furthermore, qualitative gross motor activity was recorded using an actiwatch during these 5 days. Participants also completed four test sessions of personal digital assistant (PDA) based psychomotor vigilance task (PVT) test: pre-sleep, post-sleep, pre-work, and post-work. A comparison of data was performed using the Wilcoxon signed-rank test. RESULTS: Sailors received approximately 1 h less of rest while sailing. There was a significant difference in bedtime, wake-up time, total sleep time, sleep onset latency (SOL), wake up after sleep onset (WASO), and the number of awakenings during the non-sailing and the sailing days. PVT data showed significantly higher reaction times during the sailing days on pre-work and post-sleep sessions. CONCLUSION: Our study shows that although the sailors are not getting sufficient restorative sleep while sailing as compared to non-sailing days, this is not affecting their cognitive functioning while working.

Vibronic Coupling in the First Five Electronic States of Dicyanodiacetylene Radical Cation.

We examine vibronic coupling in the first five electronic states (X̃2Πg-Ã2Πu-B̃2Σg+-C̃2Σu+-D̃2Πg-E2Πu) of dicyanodiacetylene radical cation (C6N2•+) in this article. Prompted by the prediction of its existence in the astrophysical environment, the vibronic band structure of these electronic states of C6N2•+ has been probed in spectroscopic measurements in laboratory by various groups. Inspired by numerous experimental data, we undertook the task of investigating topographical details of electronic potential energy surfaces, their coupling mechanism and nuclear dynamics on them. The degenerate Π electronic states of this radical are prone to Renner-Teller instability, and in addition symmetry allowed Σ-Π and Π-Π vibronic coupling is expected to play crucial role in the detailed vibronic structure of each of the above electronic states. A vibronic coupling model is developed here and first-principles nuclear dynamics study is carried out employing quantum mechanical methods. The vibronic band structure thus calculated is compared with experimental results and the progressions are identified and assigned. The nonradiative internal conversion dynamics among electronic states is also examined and discussed in relation to the various coupling of electronic states.

Photophysics of phenol and pentafluorophenol: The role of nonadiabaticity in the optical transition to the lowest bright 1ππ* state.

We report multimode vibronic coupling of the energetically low-lying electronic states of phenol and pentafluorophenol in this article. First principles nuclear dynamics calculations are carried out to elucidate the optical absorption spectrum of both of the molecules. This is motivated by the recent experimental measurements [S. Karmakar et al., J. Chem. Phys. 142, 184303 (2015)] on these systems. Diabatic vibronic coupling models are developed with the aid of adiabatic electronic energies calculated ab initio by the equation of motion coupled cluster quantum chemistry method. A nuclear dynamics study on the constructed electronic states is carried out by both the time-independent and time-dependent quantum mechanical methods. It is found that the nature of low-energy πσ* transition changes, and in pentafluorophenol the energy of the first two 1πσ* states, is lowered by about half an eV (vertically, relative to those in phenol), and they become energetically close to the optically bright first excited 1ππ* (S1) state. This results in strong vibronic coupling and multiple multi-state conical intersections among the ππ* and πσ* electronic states of pentafluorophenol. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the 1ππ* state is examined at length. The structured vibronic band of phenol becomes structureless in pentafluorophenol. The theoretical results are found to be in good accord with the experimental finding at both high energy resolution and low energy resolution.

The HIV/AIDS epidemic among young people in China between 2005 and 2012: results of a spatial temporal analysis.

OBJECTIVES: Despite a recent increase in the HIV/AIDS epidemic among young people in China, youth-specific HIV data are limited. METHODS: In total, 56 621 individuals with HIV/AIDS, aged 15-24 years, registered in the Case Reporting System of China between 2005 and 2012 and having complete spatial information were included in the present analysis. Spatial autocorrelation (general and local) and space-time scanning were performed using the ArcGIS 10.2 and SaTScan 9.3 software, respectively. RESULTS: During 2005-2012, the number of reported HIV/AIDS cases and the proportion of HIV cases increased while the proportion of AIDS cases decreased. Sexual contact became the predominant route of transmission in later years. Spatial analysis showed marked geographical variations in HIV infection among young people throughout China during 2005-2012. The number of new hotspots increased over time. They were mainly localized to southeastern coastal areas, southwestern frontier provinces or autonomous regions (of Guangxi, Yunnan and Sichuan) and Beijing municipality. Later these hotspots disappeared and new hotspots were found in the northeast of the country. Significant clusters of HIV-positive cases were identified in three different time periods, which indicated high HIV transmission among young Chinese people in the recent past. The risk of HIV infection was highest in the first cluster (2009-2012; this cluster was the largest in size) covering the provinces of Guizhou and Yunnan, the Chongqing municipality, Guangxi, and the province of Sichuan. The second cluster (2010-2012) was mostly located in Shanghai, South Jiangsu, Zhejiang and South Anhui, while the third cluster (2010-2012) was located in Beijing and Tianjin. CONCLUSIONS: Target-specific comprehensive behavioural interventions are urgently needed to contain the HIV epidemic among young people.