Glow discharge can be produced in dielectric barrier discharge (DBD) in helium containing impurities of the air at atmospheric pressure, which is usually interpreted qualitatively that "Penning ionization" lowers the gas discharge voltage, but it is the lack of experimental verification. This paper investigates the discharge in air and the effect of impurities of the air to helium DBDs at atmospheric pressure in different background pressure of air changing from 1 Pa to 1000Pa by measuring the emission spectra from the discharge gap. Results indicate that all the emission spectra are observed from the second positive system of N2 for a DBD in air at atmospheric pressure. When atmospheric pressure DBDs happen in helium with different background pressure of air, the light intensity of emission spectrum observed from the strong rotational band N2+ at 391.4 nm is varied by different background pressure of air, with the light intensities of emission spectra of helium decreasing. The light intensity at 391.4 nm has a peak around 190 Pa and gradually drops to lower value after 190Pa, dropping to 0 in the end. The lower gas discharge voltage is the result of Penning ionization due to helium metastables; The changes of the light intensity at 391.4 nm is the result of both Penning ionization and the quenching to helium metastables caused by N2. Besides, the changes of the light intensities of all emission spectra of helium are due to the quenching.
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