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Superconducting magnet based Faraday rotation spectrometer for real time in-situ measurement of OH radicals at 10^6 molecule/cm^3 level in an atmospheric simulation chamber Weixiong Zhao, Bo Fang, Xiaoxiao Lin, Yan-Bo Gai, Wei-Jun Zhang, Wenge Chen, Zhiyou Chen, Haifeng Zhang, and Weidong Chen Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04949 • Publication Date (Web): 28 Feb 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Analytical Chemistry

Superconducting magnet based Faraday rotation spectrometer for real time in-situ measurement of OH radicals at 106 molecule/cm3 level in an atmospheric simulation chamber Weixiong Zhao,*,† Bo Fang,† Xiaoxiao Lin,† Yanbo Gai,† Weijun Zhang,*,†, ‡ Wenge Chen,§ Zhiyou Chen,§ Haifeng Zhang,∥ Weidong Chen⊥ † Laboratory of Atmospheric Physico-Chemistry, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei, 230031, Anhui, China ‡ School of Environmental Science and Optoelectronic Technology, University of Science and Technology of China, Hefei, 230026, Anhui, China § High Magnetic Field Laboratory, Chinese Academy of Science, Hefei 230031, Anhui, China

∥Vacree Technologies Co., Ltd., Hefei 230088, Anhui, China ⊥ Laboratoire de Physicochimie de l'Atmosphère, Université du Littoral Côte d'Opale, 59140 Dunkerque, France

Corresponding Author

* Tel.: +86-551-65591961. Fax: +86-551-65591560 E-mail: [email protected], [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Atmospheric simulation chambers play vital roles in validation of chemical mechanisms and act as a bridge between field measurements and modeling. Chambers operating at atmospheric levels of OH radicals (106-107 molecule/cm3) can significantly enhance the possibility for investigating the discrepancies between the observation and model predications. However, few chambers can directly detect chamber OH radical at ambient levels. In this paper, we report on the first combination of a superconducting magnet with mid-infrared Faraday rotation spectroscopy (FRS) for real time in-situ measurement of OH concentration in an atmospheric simulation chamber. By using a multipass enhanced FRS, a detection limit of 3.2×106 OH/cm3 (2σ, 4s) was achieved with an absorption pathlength of 108 m. The developed FRS system provided a unique, selfcalibrated analytical instrument for in-situ direct measurement of chamber OH concentration.


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The hydroxyl radical (OH), primarily formed from the reaction of water vapour with O(1D) atoms by ultraviolet photolysis of ozone, plays a crucial role in the degradation of trace gases and pollutants in the troposphere and in controlling the atmospheric oxidation capacity.1-3 The concentration of OH is a measure of atmospheric self-cleansing, and can be enhanced by its regeneration in the oxidation chain of reaction products.2 The budget is generally controlled by in-situ chemistry, which makes OH an ideal target for the validation of atmospheric models through the comparison between observations and model predications. Recently, unexpectedly high OH concentrations observed in the field highlight the urgent need for more detailed studies of OH recycling mechanisms.3-7 Because of the complexity of atmospheric chemistry, atmospheric simulation chambers or photochemical reactors, in which both physical conditions and chemical compositions can be well controlled, play vital roles in validation of chemical mechanisms and act as a bridge between field measurements and modeling.8-10 However, significant discrepancies remain between high quality chamber datasets and the updated oxidation mechanism: current photochemical mechanisms greatly underestimate chamber OH concentrations.11,12 As direct measurement of OH is so difficult,13 the OH concentrations in chamber were usually determined from hydrocarbon decay using a steady state approach, and experiments were usually performed at high radical concentrations. As increasing OH concentration will promote the otherwise unlikely radical-radical reactions in the atmosphere, mechanisms developed under these conditions may not be represent the real atmosphere.10,14 Chambers equipped with highly sensitive OH measurement instruments and operating under low radical concentrations can significantly enhance the possibility for investigating these discrepancies.9,10 But so far, only two techniques - FAGE (fluorescence assay by gaseous expansion) and DOAS (differential optical absorption spectroscopy) - have been successfully used for chamber OH detection at ambient levels (106-107 molecule/cm3).9,15,16 For instance, FAGE has been used in the indoor HIRAC (Instrumented Reactor for Atmospheric Chemistry) chamber in Leeds, United Kingdom, outdoor SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) chamber in Jülich, Germany, outdoor EUPHORE (European PHOtoREactor) chamber in Valencia, Spain, etc.;17,18 while DOAS has only been used in the outdoor SAPHIR chamber.19,20 The typical detection limits (signal to noise ratio,


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SNR = 2) were 0.5 - 2.5×106 molecule/cm3 (30 - 137 s) and 1×106 molecule/cm3 (135 s) for FAGE and for DOAS, respectively.16 There are a large number of small-scale chambers are now operating around the world as an indispensable complementarily to the large-volume chambers.18 Both FAGE and DOAS are well established measurement techniques;21-24 have been validated by formal blind intercomparison in large chambers,25-28 but have some limitations in small-scale chamber applications.9 FAGE is based on laser induced fluorescence, which is not an absolute method and requires a calibration with OH of known concentration. FAGE measurements need to continuously sample the chamber gas (with a flow rate of about 2 L min-1 through the sampling pinhole).10 OH radical will lose in the sampling tube, and a replenishment gas flow is needed to compensate for the sampling to keep the chamber pressure in constant.9,16 DOAS detects OH directly by measuring its unique and structured absorption in the UV spectral range, which is an absolute method and the concentration of OH can be calculated with known absorption pathlength and absorption cross section. However, very long physical pathlengths are needed to achieve the required detection sensitivity (in the exceptionally large SAPHIR chamber, the 20 m distance between multi-path mirrors gives a total effective pathlength of 2.24 km). In addition, other species that have unstructured UV spectra are much more abundant and will absorb in this spectral region, which also increases the complexity of data retrieval and raises potential interferences for accurate measurement of OH concentrations.9,16 Faraday rotation spectroscopy (FRS) relies on the particular magneto-optic effect observed for paramagnetic species (including most radicals and some compounds with unpaired electrons), which can significantly reduce excess laser noise and makes it capable of enhancing the detection sensitivity and mitigation of spectral interferences from diamagnetic species in the atmosphere.29-37 FRS was firstly used for OH detection in 1980 with a 2.69 µm (~ 3708 cm-1) color center laser.29 The reported detection limit was about 1011 molecule/cm3.30 With the progresses of high performance diode laser and sensitive room temperature detectors, Zhao et al. achieved a 1σ detection limit of 3×108 OH/cm3 in 25 cm absorption pathlength by using a 2.8 µm distributed feed-back (DFB) diode laser.34,35 The highest absorption line intensity and the largest gJ factor (rotational gyromagnetic ratio) make the Q(1.5) double lines at ~ 3568 cm-1 clearly the best choice for FRS detection in the infrared region. The detection sensitivity of the AC (Alternating Current) longitudinal magnetic field system was 2.4×10-9 cm-1, which corresponding


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to a minimum detectable Faraday rotation angle of 4.7×10-8 rad/Hz1/2. The results showed high potential of FRS for future applications in photoreactor chambers by implementing long absorption pathlength with multipass cell. For FRS chamber application, large diameter magnet is required. The commonly used solenoid coil wrapped by enamel-copper wire can not meet the requirement due to the high winding resistance. In this paper, we report the first development of a superconducting magnet based Faraday rotation spectrometer operating at 2.8 µm for real time in-situ measurement of OH in chamber. The combination of a static magnetic field (DC-field) with laser wavelength modulation spectroscopy (WMS) was used, which provided an alternative method to effectively modulate the Zeeman splitting of the absorption lines with excellent performance.36,38 In this work, a detection limit of 3.2×106 molecule/cm3 (2σ, 4s) was achieved using a multipass cell with an absorption pathlength of 108 m. The developed system provides a suitable self-calibrated analytical instrument for in-situ direct measurement of absolute concentration of OH for study of the OH-based atmospheric oxidation process in the chamber.

EXPERIMENTAL SECTION The experimental setup and the corresponding photograph of the superconducting magnet based FRS is schematically shown in Fig. 1. A 2.8 µm continuous-wave (CW) distributed feedback (DFB) diode laser (nanoplus Gmbh) operating at room temperature was used for probing the Faraday rotation effects via measurement of the Q(1.5e) line of OH at 3568.52 cm-1. The laser temperature and current were controlled by a laser diode controller (LDC 501, Stanford Research). DC-field associated with WMS at 10 kHz (via a sinusoidal modulation signal to modulate the laser injection current) were used to effectively modulate the Zeeman splitting of the absorption lines. A 10 Hz ramp generated from a function generation (Agilent 33521B) was used to sweep the laser. The laser output was collimated by an aspheric lens with an effective focal length of 4 mm and a diameter of 6.5 mm (Edmund Optics). The collimated laser beam (~ 6 mm in diameter) passed through the first linear polarizer (Rochon prism, with an extinction ratio of ξ < 5×10-6, Foctek Photonics) to establish a polarization axis of the incident laser, and then directed into the multipass cell, located inside a quartz reaction chamber and a superconducting magnet. The


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second, identical Rochon prism, placed between the multipass cell and photodetectors, acts as a polarization analyzer. A differential detection scheme was used to suppress the laser noise and optical fringes.39,40 The polarization axis of the analyzer is set at 45° angle with respect to the polarization of the incident light to achieve the largest FRS signal. Light exiting the multipass cell was split by the analyzer into two orthogonal polarization components that were detected with two thermoelectrically cooled (HgCdZn)Te photovoltaic detectors having the same voltage responsivity (referred to as signal and reference detectors in Fig. 1, respectively). These two signals, with equal laser intensity but opposite signs of the FRS signals, were demodulated with a lock-in amplifier with differential (A-B) model, which resulted in a doubled FRS signal amplitude at a reduced effective noise level. The demodulated FRS signal from the lock-in was then digitalized and acquired with a DAQ card (NI PCIe-6351).

Figure 1 Schematic diagram and photograph of the superconducting magnet based FRS system. Both polarizer and analyzer were Rochon type. F: lens, DAQ: data acquisition, PC: personal computer.

The cylinder reaction chamber was made of a 1535 mm long quartz tube with an inner diameter of 270 mm and a wall thickness of 5 mm (Supporting Information Figure S1). The volume and inner surface area of the reactor were ~ 80 L and 1.46 m2, respectively, giving a surface/volume ratio of ~ 18.25 m-1. The chamber can be pumped from ambient pressure to ~ 0.6


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Pa. The leak rate of the chamber is about 2×10-7 bar s-1. Six UV-C lamps (~ 1500 mm in length, 16 mm diameter), which have a strong emission at 254 nm, were used as the photolysis light source to produce OH in the chamber via ozone photolysis. These lamps were housed in quartz tubes (~ 1700 mm long, with 28 mm inner diameter and 1.5 mm thickness) that were parallel to the chamber axis, and equally spaced around the circumference of a circle with a diameter of 234 mm. Compressed filtered air was used to cool down the UV lamps and remove any ozone formed for safety. A modified Chernin type multipass cell41,42 with three objective mirrors and two field mirrors was used to increase the absorption pathlength. A photograph of the optical system is shown in supporting information Figure S2 (a) and (b). The dimensions of the two field mirrors and three objectives were 75 mm × 15 mm, 105 mm × 90 mm, and 45 mm diameter, respectively. All mirrors have the same radii of curvature (ROC = 1500 mm). The mirrors were coated with protected silver for a reflectivity of ~ 97%. The five mirrors' configuration was easy to align, with very good stability to vibrations, and gave variable rows with even columns images on the field mirrors. Compared to typical White type cells, this configuration offers the most efficient use of the field mirror. Different spot patterns are shown in supporting information Figure S2: 7 rows × 4 columns (c); 6 rows × 6 columns (d); 7 rows × 6 columns (e); 8 rows × 8 columns (f). For our system, the distance between the field and object mirrors was 1.5 m. These spot patterns correspond to absorption pathlengths of 84 m, 108 m, 126 m, and 192 m, respectively. After considering transmission intensity of the probe laser, the optimum pathlength was set at 108 m in this work. The magnet coil was made of NbTi superconducting wires wrapped around a copper sheathing steel pipe (1190 mm long and 550 mm outer diameter). The coil was sealed in a Dewar vessel and cooled with a Gifford-McMahon (GM) cycle cryocooler with He from Sumitomo Industries (SHI F-50, 4.2 K) below 5 K to meet the superconducting state. The mass of the whole superconducting magnet system was about 700 Kg. The length of the magnet was 1280 mm with an inner diameter of 500 mm and an outer diameter of 760 mm. The magnet was operated in DC mode and the magnetic intensity was adjustable (17.9 Gauss/A) with a resolution of 2 Gauss. The spatial distribution of the magnetic field strength (B) inside the magnetic is shown in Supporting Information Figure S3. A circular column (with 100 cm long and 40 cm diameter)


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uniform magnetic field strength distribution (within ~ 3%) was achieved. The uniformity makes it easy to accurately mode FRS spectra.

RESULTS AND DISCUSSION For the performance evaluation of our system, chamber OH was generated by the photolysis of O3 with 254 nm UV-C lamps via following the reactions:8 O3 + hν (< 300 nm) → O(1D) + O 2 , O(1D) + H 2 O → 2OH

(1)

The OH concentration inside the chamber was determined by a WMS 2f detection scheme (as shown in Fig. 2). In the present work, all the WMS and FRS spectra were measured with a 4 s acquisition time at a 10 Hz scan rate and 40-sweep average with a lock-in time constant of 1 ms. Under the same sampling pressure and pathlength, a reference gas with known concentration (H2O in this work) in the same spectral region can be used for cross-calibration of the sample gases of interest (OH in this work).34,43,44 H2O absorption at 3568.54995 cm-1 and OH absorption at 3568.5238 cm-1 were used for the determination of chamber OH concentration. The OH concentration ([OH]) can be expressed as the ratio of the peak-to-peak 2f signals at line centre after normalization to the laser power:34,43

[OH]=

S2 f (ν 0 )OH ( I 0 )OH SH 2O S2 f (ν 0 ) H2O ( I 0 ) H 2O SOH

[H 2O]

(2)

where S2 f (ν 0 )OH and S 2 f (ν 0 ) H 2O are the WMS 2f peak signals, ( I 0 )OH and ( I 0 )H2O are the laser power at the center wavelength of OH and H2O. SOH and S H 2O are the absorption line strengths of OH and H2O with a value of 9.454×10-20 cm-1/(molecule cm-2) and 2.824×10-26 cm1

/(molecule cm-2) at 296 K taken from the HITRAN (HIgh-resolution TRANsmission molecular

absorption) database,45 respectively. H2O concentration ([H2O]) was determined from the direct absorption of H2O. In this analysis, the instrument dependent parameters, such as the second harmonic transfer function of the lock-in amplifier, photodiode responsivity, etc., all can be canceled out.43 The accuracy of OH mixing ratio is mainly determined by the uncertainty in the HITRAN available line strengths of H2O (< 2%) and OH (~ 1%), and the experimental uncertainty in the determination of H2O concentration by direct absorption spectroscopy (DAS) (< 4%). The total uncertainty (summed in quadrature) of the OH source concentration was estimated to < 5%.


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WMS 2f signal (V)

2.0

At the beginning, 0 s, Lamp off Lamp on, 16 s DC output of the signal detector

(a)

1.5 1.0

0.25

0.20

0.5 0.15

0.0 0.10

-0.5 -1.0

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WMS 2f signal (V)

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0.05

-1

H2O @ 3568.54995 cm

-1

(b)

Laser Intensity (V)

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OH @ 3568.5238 cm

Substraction of H2O absorption

0.2 0.1 0.0 -0.1 -0.2 -0.3 -1

OH @ 3568.4169 cm

-0.4 -0.5 0

2000

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10000

Data points

Figure 2 (a) WMS 2f spectra of OH and H2O absorption during the photolysis of O3. At the beginning of the photolytic process, the UV lamps were turned off. After turning on the lamps, OH absorption can be clearly observed. A spectrum at a time of 16 s after the lamps were turned on was shown as an example for the determination of OH concentration. (b) To make OH absorption clear, H2O absorption at the beginning was subtracted from the photolytic spectrum. The absorption lines of H2O at 3568.54995 cm-1 and OH at 3568.5238 cm-1 in the magenta dash box were used for the cross-calibration of chamber OH concentration. The laser power as a function of wavelength used to normalize the WMS spectra is shown in the blue line in (a). The reduction of laser intensity was due to ambient H2O absorption. All the measurement was performed at a chamber pressure of 50 mbar. The measured H2O and OH concentrations were 1.53×1016 molecule/cm3 and 2.28×109 molecule/cm3 respectively. In FRS system, for weak absorption, light transmission of the analyzer is given by:29,34

P(ϕ ) =

P0 (1 − cos 2ϕ + R∆ L sin 2ϕ ) 2

(3)

where P0 is the incident power of the laser; φ is the cross angle between two polarizers; L is the absorption pathlength; R∆ is responsible for the generation of FRS signal, which can be calculated with:34

R∆ = k0 (n+ − n− ) =

NS ln 2

πγ D

∑ ( −1)

M J' , M J''

M J' − M J''

 ln 2    ' ' '' '' µ B B Re Z  − − g M − g M + i ν ν γ ( ) ( ) 0 J J J J C    hc  γ D   (4)


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where k0 is the wave vector, n+ and n- are the refraction indices of the sample for right-handed (RHCP, +) and left-handed (LHCP, -) circularly polarized waves, N and S are the molecule concentration and absorption line intensity, respectively. M' and M'' are the magnetic quantum numbers for the upper and lower states. Z is the plasma dispersion function. ν0, γC and γD are the absorption line center wavenumber, the collisional and Doppler half width, respectively. µB, B, h and c are respectively the Bohr magneton, the magnetic-field strength, the Planck constant, and the speed of light in vacuum. To maximize the FRS signal, a series of experiments at a chamber pressure of 50 mbar were performed to determine the optimum magnetic field strength Bopt (as shown in the Supporting Information Figure S4). The largest FRS signal was found at a current of 12 A. Under superconducting conditions, the corresponding load voltage of the magnet was ~ 0.05 V, which corresponded to Bopt = 215 Gauss. All experiments were then performed under these conditions.

0.3

(a)

A, Signal detector

(b)

B, Reference detector

(c)

A-B, Differential detection

0.0 -0.3 -0.6

FRS signal (a.u.)

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0.6 0.3 0.0 -0.3

0.5 0.0 -0.5 -1.0 0

1000

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10000

Datapoints

Figure 3 The 2f DC-FRS signal of the two orthogonal polarization components: signal beam (a) and reference beam (b). The signal resulting from dual detectors with a balanced detection scheme is shown in (c). The OH signals are masked with olive dash box, and baselines containing H2O absorption are masked with magenta dash box.


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A typical 2f DC-FRS spectrum of OH at 3568.5238 cm-1 with differential detection scheme is shown in Fig. 3(c). The FRS signals of the two orthogonal polarization components recorded with the signal detector (Fig. 3(a)) and reference detector (Fig. 3(b)) have opposite signs, but have the same baselines that contained H2O absorptions and optical fringes. The balanced detection scheme shows clearly noticeable advantages in cancelling of laser noise and the variations in the laser intensity resulting from strong absorption caused by diamagnetic species and 2 times increment of the FRS signal, despite the possible deterioration of the polarization state after multiple reflections off the mirror surfaces.39,40 FRS is based on the measurement of optical dispersion, which is linear with concentration in a large dynamic range.40 Fig. 4 shows the relation between FRS signal and OH concentrations, which were determined from the method described above. The FRS signal was linearly proportional to the OH concentration: [OH] (molecule/cm3) = 4.551 (±0.163)×106 (molecule/cm3) + 1.945 (± 0.055)×109 (molecule/cm3/mV) × FRS signal (mV). The linear fit uncertainty of the calibration curve was less than 3%. 1 mV FRS signal corresponded to a concentration of 1.945 ×

3

109 molecule/cm3.

[OH] molecule/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1x10

9

1x10

9

9x10

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8x10

8

7x10

8

6x10

8

5x10

8

4x10

8

3x10

8

2x10

8

1x10

8

6

9

y = 4.551 (±0.163)10 + 1.945(±0.055)10 x 2 R = 0.9888

0 0.0

0.1

0.2

0.3

0.4

0.5

FRS signal (mV) Figure 4 The OH concentrations versus the measured FRS signal intensities.


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To characterize the photooxidation process of O3 and evaluate the performance of current FRS system, FRS spectra of OH were recorded at different photolysis time (Fig. 5). The OH concentration decayed with consumption of O3. At a time of 132 s, O3 was almost exhausted; however, the FRS absorption spectrum can be clearly observed compared with the background signal at the beginning of the photolysis when the UV lamps were turned off. The 1σ standard deviation values of FRS background signal intensity at the beginning of the photolysis was 0.8 µV, which corresponding to a limit of detection of OH radical of 1.6×106 molecule/cm3 with 4 s acquisition time. The minimal detectable fractional absorption (∆Pmin/P0) of FRS is about two orders lower than that of DOAS. The achieved OH detection limit is comparable to that of Jülich FAGE and DOAS instruments (as shown in Table 1). Due to the high reactivity of OH radicals, a standard sample with constant OH concentration is not available and the Allan variance plot was accordingly not studied in in this work. However, improvements in the signal-to-noise ratio are expected with increasing averaging time.

0.020

(b)

(a)

0.04

At the beginning, 0 s, Lamp off Lamp on 112 s Lamp on 132 s

0.03

0.015

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0.00 0.005 -0.01 -0.02

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FRS signal (mV)

0.010

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FRS signal (mV)

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-0.03

1σ σ ~ 0.8 µV

-0.04

Lamp on 72 s Lamp on 80 s Lamp on 88 s Lamp on 96 s Lamp on 104 s Lamp on 112 s

-0.05 -0.06 -0.07 0

1000

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-0.005

LOD[OH] ~ 1.6 × 106 cm-3 -0.010

3000

Datapoints

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5000

0

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-0.015 5000

Datapoints

Figure 5 Evolution of OH FRS signals in the O3 photolytic process. (a) FRS signals at different photolysis time. (b) FRS signals close to the end of the photolytic process when O3 was almost exhausted. The background at the beginning of the photolysis was also shown to clearly determine the detection limit of current system.


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Table 1 Comparison of the performance of the developed superconducting magnet based FRS in this work with that of the Jülich FAGE and DOAS instruments. For FAGE, which is not based on the Beer-Lambert law, items related to absorption spectroscopy are not listed.

Method FAGEa DOASa FRS a

Data acquisition time (s) 47 205 4

Detection Limit (106 molecule/cm3) 0.3 0.5 1.6

Minimal detectable fractional absorption NA 1×10-5 1.4×10-7

Absorption pathlength (m) NA 2240 108

Minimal detectable fractional absorption scaled to pathlength (cm-1) NA 4.5×10-11 1.2×10-11

adapted from Ref. 28.

CONCLUSIONS We have developed a superconducting magnet based Faraday rotation spectrometer operating at 2.8 µm for chamber OH detection. A 1σ detection limit of 1.6×106 molecule/cm3 was achieved with an absorption pathlength of 108 m and 4 s data acquisition time. The performance of the spectrometer can be further improved by using highly polished optical substrates with higher reflectivity dielectrically coated mirrors to increase the absorption pathlength and signal intensity,46 or by employing cavity ring-down scheme.47,48 The uniform distribution of B inside the chamber makes it possible to apply a fast curve fitting algorithm to enable real-time accurate concentration retrieval.49 In addition, with adjustable magnetic field and absorption pathlength, a survey of the key parameters for a particular instrument development can be done with our Faraday rotation system. The application of the superconducting magnet system is not limited to OH radical, but could also be used for NO, HO2, and other paramagnetic radicals. Compared with previously reported FRS measurement NO33 and HO237 with short absorption pathlengths, the detection limits of these radicals are expected to be improved by two to three orders of magnitude. Currently, only O3 photolysis was done for the demonstration performance of the developed system. By changing the UV-C light source to UV-A or sun lamps,8 more experimental systems can be investigated. As a new method, intercomparison studies, with some well-developed instruments like FAGE, or calculated OH concentration with the decay of a hydrocarbon are necessary and will be done in the future for careful assessment of the accuracy of the FRS


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system.11 In general, the developed superconducting system will provide a unique platform for atmospheric chemistry research.

ASSOCIATED CONTENT Supporting Information Photograph of the quartz cylinder photochemical reactor, the mirrors and spot patterns of Chernin multipass cell, distribution of magnetic field strength inside the superconducting magnet, and optimum magenetic field strength for the measurement.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research is supported by the National Natural Science Foundation of China (41127001, 91544228, 41627810), the Natural Science Foundation of Anhui Province (1508085J03), and the Youth Innovation Promotion Association CAS (2016383). We thank colleagues in High Magnetic Field Laboratory, CAS and Vacree Technologies for their work in constructing the superconducting magnet. Weixiong Zhao would like to thank Profs. Dawyne E. Heard, Paul W. Seakins, Dr. Mark A. Blitz, and members of their groups for helpful discussion during his visit of Leeds University in June 2014. We thank Dean Venables (University College Cork, Ireland) for discussions on this work.

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