This motive is a variation of Desire in that the contour of the melodic line moves up by half-step intervals.  Several analysts including Pfohl and Kobbé also consider it a variation of Desire.

Motive Labels
“Hero Motive” – (Cleather)
“Tristan’s Anguish” – (Newman, in Bailey, p. 155)
“Sehnsuchts” or “Longing” – (Windsperger, p. 39)
1st measure = Motive 2 – (Pfohl, p. 216)
Variation of Motive 2 (Isolde Motive) – (Kobbé, p. 111)

Hero – Order of occurrence by Act and Scene
Act I: Prelude