SPP Olaya Revised

Collisional energy transfer between helium and neon in a glowdischarge

Cherrie May M. Olaya∗, Rommil Emperado, Joshua Beringuela, Krizia Isabel A. Lampa andWilson O. Garcia

National Institute of Physics, University of the Philippines, Diliman, Quezon City∗Corresponding author: [email protected]

AbstractOptical emission measurements were made on He, Ne and a mixtureof pre-excited He and unexcited Ne to demonstrate the energy transfermechanism between He and Ne. The emission spectra of He, Ne and HeNemixture were compared. Collisional energy transfer between the excitedstates of He and Ne were observed by the increase in the 2p55s1 lineintensity of Ne and the decrease in the 23S1 and 21S0 line intensity of He.

PACS Number: Demonstration experiments (physics education)(01.50.My), Spectral sources, electric-discharge (52.80.Yr), Spectroscopyin atomic and molecular physics (07.57.-c)

1. IntroductionHelium-neon (HeNe) lasers are CW gas lasers which are simple, reliable, and cost-efficient. As such, these

devices are among the most common and familiar types of lasers [1]. Laser action occurs in the excited statesof Ne. The purpose of He is to facilitate pumping by transferring the energy it gained from electron collisionsto the states in the unexcited Ne. Figure 1 shows the relevant energy transitions in the lasing action of aHeNe laser [1,2].

Figure 1: Relevant energy levels of the HeNe laser [3]

Electrons are accelerated across the HeNe laser tube by a potential difference. He atoms, upon collisionswith the electrons, are excited to various states. Among these states are the metastable 23S1 (τ = 19.5 ms)and 21S0 (τ = 104 s) states [4]. These states are nearly resonant with the 2p54s1 and 2p55s1 excited states

32nd Physics Congress of the Samahang Pisika ng PilipinasUniversity of the Philippines, Diliman, Quezon City

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Table 1: Laser transitions of a HeNe laser in the visible region [6]Transition Wavelength (nm)

2p5 → 5s1 3s2 → 2p1 730.52p5 → 5s1 3s2 → 2p2 640.12p5 → 5s1 3s2 → 2p3 635.22p5 → 5s1 3s2 → 2p4 632.82p5 → 5s1 3s2 → 2p5 629.42p5 → 5s1 3s2 → 2p6 611.82p5 → 5s1 3s2 → 2p7 604.62p5 → 5s1 3s2 → 2p8 593.92p5 → 5s1 3s2 → 2p10 543.4

of Ne, respectively. Energy is allowed to transfer by collision from the 23S1 state of He to the 2p54s1 stateof Ne and from the 21S0 state of He to the 2p55s1 state of Ne. Since the excited states of He are metastablestates, population inversion is ensured. Laser transition occurs with the decay of the 2p54s1 and 2p55s1

levels of Ne to 2p53p1 and 2p54p1 levels [2,3,5]. Table 1 shows different laser transitions of a HeNe laserwithin the visible region and their corresponding emission wavelength.

Conventionally, construction of HeNe lasers have been conducted by mixing He and Ne gases before elec-trical pumping. In this work, pre-excited He was mixed with ground state Ne in an attempt to demonstratethe transfer of energy from He to Ne through collisions between excited He and ground state Ne. Thisstudy would provide further insights in the lasing mechanism of a HeNe laser and may be extended to aclassroom-level experiment tackling the energy transfer between He and Ne in the operation of a HeNe laser.

2. Experimental DetailsThe schematic diagram of the experimental is shown in Figure 2a. It consists of an L-shaped tube as

shown in Figure 2b. He (99.9999% purity) and Ne (99.9999% purity) gases are introduced to the systemthrough end 1 and end 2, respectively. A rotary pump is connected to end 3 to pump the gas out of thetube. A vacuum gauge was used to monitor the pressure of the gas. The gas discharge was excited by anSRS PS 325 high-voltage DC power supply connected to an 80kΩ resistance to limit the current. The lightemission at the negative glow region of the discharge was collected with a fiber bundle connected to OceanOptics HR2000+ES spectrometer with a resolution of ∼ 1.33 nm.

Figure 2: Schematic diagram of the experimental setup.

Emission measurements of pure He gas were obtained through the fiber bundle that was probed at thenegative glow region of discharge tube A. Likewise, discharge tube B was used to obtain emission measure-ments for pure Ne gas. Emission measurements for the pre-excited He and Ne mixture were conducted bysupplying voltage through discharge tube A and probing the fiber bundle at discharge tube B.



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3. Results and Discussion

Figure 3: Emission spectra of (a) pure He glow discharge at V = 1250 V and (b) pure Ne glow discharge atV = 1100 V.

Figure 3 shows the emission spectra of pure He and pure Ne which were probed on the negative glowregion of the discharge. Ten He (I) peaks were observed for the pure He emission. Of these peaks, threeemission wavelengths correspond to the metastable level of He used for energy transfer in HeNe lasers: 357.89nm (3S), 393.85 nm (1S), and 706.08 nm (3S). For the pure Ne emission spectra, sixteen Ne (I) emissionpeaks were observed, five of which correspond to the emission from the 2p55s1 excited state of Ne. Thesefive emission wavelengths are 593.39 nm, 606.60 nm, 632.50 nm, 639.30 nm and 659.22 nm.

Figure 4: Emission spectrum of pre-excited He and unexcited Ne at V = 1250 V.

Figure 4 shows the emission spectrum when pre-excited He is mixed with unexcited Ne. Emission fromboth He (I) and Ne (I) were observed. The presence of Ne (I) in the spectrum showed that the injected Negas in the system was excited either by the accelerated free electrons in the discharge tube or from energy



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transfer from the excitation of He.The presence of 2p55s1 emission in the spectrum for pure Ne showed that excitation to this level is

still possible from the direct electron-Ne collision. But this excitation is not dominant as seen in the lowerintensities in these wavelengths. With the introduction of excited He in the gas system, the energy from the23S1 and 21S0 level of He is transferred to the 2p55s1 level of Ne through collisions. This energy transferis evident with the increase in intensity of the 2p55s1 emission wavelengths of Ne in the HeNe mixture, andwith the decrease in intensity of the 23S1 and 21S0 emission wavelengths of He in the HeNe mixture. Thetransfer of energy can be symbolically written as:

He∗ +Ne→ He+Ne∗ ±∆E (1)

where ∆E is the energy defect due to the excited levels of He and Ne not being precisely resonant [6].

4. ConclusionEmission spectroscopy was used in demonstrating the collisional energy transfer from He to Ne in the

discharge from a mixture of pre-excited He and Ne. Perpendicularly oriented discharge tube was used toallow pre-excitation of He before mixing with unexcited Ne. Increase in the line intensity of 2p55s1 level ofNe and decreased line intensity of 23S1 and 21S0 levels of He in the HeNe mixture showed transfer of energythrough collisions allowing the lasing energy levels of Ne to populate.

References[1] J. Hollas, Modern Spectroscopy. West Sussex, England: John Wiley Sons, Ltd, fourth edition ed., 2004.

[2] P. Milonni and J. Eberly, Lasers. USA: John Wiley Sons, Ltd, 1988.

[3] O. Svelto, Principles of Lasers. New York, USA: Plenum Press, fourth edition ed., 1998.

[4] R. V. Dyck, C. Johnson, and H. Shugart, “Radiative Lifetime of the 21So Metastable State of Helium,”Physicsal Review A, vol. 4, no. 4, pp. 1327–1336, 1971.

[5] A. Siegman, Lasers. USA: University Science Books, 1986.

[6] J. Verdeyen, Laser Electronics. Englewood Cliffs, New Jersey: Prentice-Hall, Inc., third edition ed., 1995.



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SPP Olaya Revised