3b-scientific Teltron Critical Potentials Tube S with He-Filling User Manual download pdf

Atomic and Nuclear Physics

Atomic Shells

1 / 3

Critical Potentials

DETERMINE THE CRITICAL POTENTIALS OF A HELIUM ATOM.

• Measure the collector current I

as a function of the accelerating voltage U

• Compare the positions of the current maxima with the known critical potentials of the helium atom.

• Identify the doublet structure in the term scheme of helium (ortho and para helium).

UE502050

08/08 JS

BASIC PRINCIPLES

The expression “critical potential” is a general name for all the

excitation and ionisation energies in the electron shells of an

atom. The corresponding electronic states of the atom can be

excited in various ways, for example by inelastic collisions with

electrons. If the kinetic energy of the electron corresponds

exactly to a critical potential, the electron can transfer all its

kinetic energy to the atom in an inelastic collision. Using an

experiment set-up originally designed by Gustav Hertz, this

effect can be used to determine the critical potentials.

In a tube that has been evacuated and then filled with helium, free

electrons are accelerated by a voltage U

to form a divergent beam

passing through a space at a constant potential. To prevent the

walls of the tube becoming charged, the inner surface is coated

with a conducting material and connected to the anode A (see Fig.

1). In the tube there is a ring-shaped collector electrode R, through

which the divergent beam can pass without touching it, even

though the ring is at a slightly higher potential.

However, a small current I

, with a value in the order of

picoamperes, is measured at the collector ring, and is found to

depend on the accelerating voltage U

. It shows characteristic

maxima, which are caused by the fact that the electrons can

undergo inelastic collisions with helium atoms during their passage

through the tube. The kinetic energy E of an electron is as follows:

(1)

UeE ⋅=

e: elementary electron charge

If this energy corresponds exactly to a critical potential of the

helium atom, all the kinetic energy may be transferred to the

helium atom. In this instance the electron can then be attracted

and collected by the collector ring, thus contributing to an

increased collector current I

As the accelerating voltage is increased, successively higher levels of

the helium atom can be excited (see the term scheme of the helium

atom shown in Fig. 2), until finally the kinetic energy of the electron

is enough to ionise the helium atom. As the accelerating voltage is

increased further, the collector current shows a steady increase.

1.5 V

Fig. 1: Schematic diagram of critical potential tube

/ eV

= 1

-5

-15

-10

-20

-2

Fig. 2: The term scheme of helium

red: total spin S = 0 (para helium),

green: total spin S = 1 (ortho helium)

1 2 3 4

Summary of Contents

Page 1 - Atomic and Nuclear Physics

Atomic and Nuclear Physics Atomic Shells 1 / 3 Critical Potentials DETERMINE THE CRITICAL POTENTIALS OF A HELIUM ATOM. • Measure the collector

Page 2 - EXPERIMENT PROCEDURE

UE502050 3B SCIENTIFIC® PHYSICS EXPERIMENT 2 / 3 LIST OF APPARATUS 1 Critical Potential Tube S with He-Filling U18560 1 Tube Holder S U185001 1

Page 3

Page 4

3b-scientific Teltron Critical Potentials Tube S with He-Filling User Manual

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3b-scientific Teltron Critical Potentials Tube S with He-Filling User Manual

Browse online or download User Manual for Health and hygiene 3b-scientific Teltron Critical Potentials Tube S with He-Filling. 3B Scientific Teltron Critical Potentials Tube S with He-Filling User Manual

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